Leonardo DiCaprio has always had an age problem. The Oscar-winning actor, who turns 50 on Monday, has spent the lion’s share of his screen career wrestling with perceptions of his seemingly permanent adolescence. He’s a former child actor who played boys and teenagers well into his twenties. He became frozen in the aspic of global celebrity as the fresh-faced Jack Dawson in the mega-smash Titanic (cue worldwide “Leomania”). And when he attempted to “transition” into grown-up roles in his thirties, it somehow didn’t seem real. On marking DiCaprio’s Oscar nomination for Blood Diamond in 2007, for instance, the great film critic Philip French noted that DiCaprio, who was then 32, was a “superb actor who hasn’t yet quite become an adult”.

His off-screen antics didn’t help. DiCaprio was frequently described as a “party boy” who liked to go on all-night benders with entourages 20-strong, rack up eye-watering bar bills and, according to an infamous New York Magazine profile from 1998, “chase girls, pick fights and not tip the waitress”.

DiCaprio’s squad at the time was dubbed “the pussy posse” for reasons, alas, not at all associated with cats. It set the template for the years to come, where “chasing girls” would evolve into a regular pattern of paramours who were invariably models (Gisele Bündchen, Bar Refaeli, Camila Morrone etc) and usually 25 or younger, even as DiCaprio happily soared towards his sixth decade. He’s pushing the romantic envelope, though, by dating the Italian model Vittoria Ceretti, who is, er, 26 years young.

This dating pattern is jokingly called “Leo’s Law” (no girlfriends over 25, even as you get older), yet it touches upon something quite profound, and very Dorian Gray, in the DiCaprio narrative. It’s as if there’s an attic somewhere full of middle-aged models and actresses who are in fact DiCaprio’s peers (Christy Turlington, say, or Olivia Colman) while he swans about in a twentysomething dating pool that speaks directly to a personal mythology that was forged in the smithy of late-1990s stardom.

Everything, in short, changed with Titanic, when the star was 21 years old. DiCaprio has spoken before about the blinding media glare that came with James Cameron’s apex blockbuster, saying that he had no connection to “that whole Titanic phenomenon and what my face became around the world”.

Before that he had been “his generation’s great acting promise” (according to the San Francisco Chronicle) who had been nominated for an Oscar at 19 for What’s Eating Gilbert Grape, and had been hand-picked by Robert De Niro to star opposite him in the coming-of-age drama This Boy’s Life.

It’s easy to underestimate the impact of the latter film on the legacy of Leo. It’s a two-hander, adapted from a novel by Tobias Wolff, and stars the inevitably “boyish” DiCaprio (17 during filming) as an alienated teen who frequently clashes with his mother’s new, fastidious and increasingly violent boyfriend, Dwight, played by De Niro.

There is a certain charge in the exchanges between De Niro and DiCaprio. It’s a thrilling, baton-passing quality that’s reminiscent of Brando handing over the keys of the Method kingdom to Al Pacino during The Godfather. That DiCaprio will eventually overtake De Niro as Martin Scorsese’s favourite muse somehow makes the film even more poignant.

This Boy’s Life, however, also established a template that recurred in DiCaprio’s career, that of working with a screen hero or older Tinseltown legend only to be wholly overshadowed by them. DiCaprio’s first big splash for Scorsese, for instance, was in Gangs of New York, where he was roundly eclipsed by the mesmerising turn of his co-star Daniel Day-Lewis.

Later, in Scorsese’s The Departed, DiCaprio piled on 15lb of muscle and created a character of searing intensity, only to be outdone by the louche theatrics of his co-star and idol Jack Nicholson. The pattern seemed to reinforce the idea that DiCaprio had not yet reached maturity and was being surpassed by the grown-ups in the room.

But, yes, back to Titanic. DiCaprio wasn’t even supposed to do that film. He was circling Boogie Nights instead and was the director Paul Thomas Anderson’s original choice for the lead. Just let that sink in.

Boogie Nights with DiCaprio as Dirk Diggler, the role eventually made famous by Mark Wahlberg? And no Titanic? It would have altered the course of his professional life and beyond. “I’m not saying I would have,” DiCaprio told GQ when discussing why he “wished” he had done the film instead of Titanic. “But it would have been a different direction, career-wise.”

The direction Titanic sent him was mostly into the hearts of young women and screaming girls, which often seemed to provoke the ire of young men and overprotective fathers. Senator John McCain, when referring to his daughter’s obsession with DiCaprio, famously called the actor “an androgynous wimp”.

It’s hardly surprising then that DiCaprio seemed to spend the entire mid-section of his career (at least until his late thirties) attempting to prove that he wasn’t just a pretty face, or indeed a pretty boy. This was sometimes unsuccessful, if not borderline farcical. In The Aviator, for instance, he hid poorly underneath a glued-on beard and old-man make-up as disturbed, late-era Howard Hughes.

Off-screen he was becoming an environmental heavy hitter, establishing the Leonardo DiCaprio Foundation and backing a plethora of eco-activist projects and documentaries including the film The 11th Hour and the series Greensburg.

In 2014 the UN secretary-general Ban Ki-moon appointed DiCaprio a UN messenger of peace with a focus on climate change. In 2016 Time magazine included him in its list of “100 most influential people in the world”.

He made some canny business decisions, including taking a pay cut to star in Inception while negotiating a share of the film’s ultimately enormous box office. This allegedly resulted in him earning $50 million for that movie alone. It might explain his not-especially-voracious appetite for screen roles, preferring instead to bash out a prestige picture every two or three years, and instead get papped while holidaying on yachts in the south of France next to, naturally, some models.

His record as a serious Hollywood player is patchy. When he gets producer credits on his titles he generally hits paydirt. But the projects with other actors that he backs are often painful flops — see Ben Affleck’s Live by Night and Runner Runner, or Taron Egerton’s Robin Hood.

Back on screen, in the interim DiCaprio was discovering his true metier as a performer. He began to excel in roles that required acting, or at least the foregrounding of the process. In The Departed his cop character Billy Costigan was “playing at” being a mobster. In Shutter Island his Teddy Daniels is deluded and pretending to himself that he’s a serving US marshal.

In The Great Gatsby DiCaprio’s title character is all front, all charade, acting like the millionaire he thinks we want to see. It’s as if DiCaprio is so saturated in his celebrity, or status as a global icon, that the only way it makes sense is to acknowledge the performer behind the performance.

This process eventually reached its apogee in Quentin Tarantino’s Once Upon a Time in Hollywood. In the most extraordinary sequence of the film, perhaps of his career, DiCaprio’s fading movie star character Rick Dalton plays the villain on a low-budget western TV series, Lancer.

The scene is a hackneyed saloon stand-off, yet DiCaprio plays it with such expressive gusto that he veers, in the space of a single monologue, from camp to crazed to complex profundity. Once “cut!” is yelled, his preteen co-star Trudi Frazer (Julia Butters) whispers earnestly into his ear: “That was the best acting I’ve ever seen in my whole life!”

Dalton’s eyes fill with tears and we know as an audience that Trudi is speaking the truth and that DiCaprio has transformed before us into something genuinely exquisite. He was rightfully Oscar- nominated for the role but was beaten by Joaquin Phoenix for his (slightly gimmicky, it now seems) turn in Joker.

Since then there’s been the environmental comedy Don’t Look Up and another Scorsese collaboration, Killers of the Flower Moon. In these DiCaprio has been fabulously shifty, nerdy and, in Killers, downright repellent.

As he segues into his fifties, in other words, he is becoming the epitome of an ambitious and nuanced character actor who has already made way for a new generation of pretty young things (see Timothée Chalamet).

And still, perhaps the greatest sign of DiCaprio’s evolution has yet to come, with his next movie. This one’s a crime film called The Battle of Baktan Cross and it marks his first creative collaboration with his “almost” Boogie Nights director, Paul Thomas Anderson.

There is sweet circularity here. Nearly 30 years later, that wished-for “what if” Boogie Nights-inspired career can finally find expression in DiCaprio. And, in his own way, as he celebrates his half-century he can put the legend of the Leomania poster boy to bed and announce: “Behold the man!” And so, yes, his “age problem” is definitely behind him these days. Now, if only he could move on from those models.

While DiCaprio is known for his acting skills and commitment to environmental activism, his dating life has become a source of ongoing fascination, particularly his penchant for dating women significantly younger than himself. His relationships with Gisele Bündchen, Bar Refaeli, Blake Lively, and Camila Morrone, all ended before they reached the age of 25. While the once baby-faced star of Titanic and The Beach may be showing his age, the twentysomething women with whom he surrounds himself most certainly are not.

DiCaprio's current girlfriend, Vittoria Ceretti, broke this “Leo’s Law” by dating him past the age of 25. DiCaprio is 49 and has been dating the 26-year-old Ceretti for over a year, prompting speculation that he has finally settled down. However, sources close to the actor say that he is not planning to have children and does not see the point in settling down. His parties are legendary, with guests ranging from Lady Gaga to Beyonce and Jay-Z to Kim Kardashian. DiCaprio is often seen surrounded by beautiful young women at these parties, quietly watching and observing, but he occasionally gets up and starts rapping, as he did at his 49th birthday party.

While DiCaprio’s dating life has generated plenty of buzz, he has also made a name for himself as one of Hollywood’s most outspoken eco-warriors. He heads his own charity, raising millions to fight global warming. Despite his commitment to environmental activism, however, his lifestyle has been the subject of criticism, particularly his use of private jets. Some argue that his commitment to the environment is undermined by his extravagant lifestyle.

Despite the controversy surrounding his dating life and his environmental activism, DiCaprio remains one of Hollywood’s most popular and successful actors. His dedication to his craft and his outspoken advocacy for climate change have cemented his status as a global icon. As he turns 50, it will be interesting to see how he continues to evolve as an actor and as an activist. Only time will tell if he will eventually settle down and start a family. Will he finally put the “Leomania” poster boy to bed? Only time will tell.