Baby-faced yet self-assured, Leonardo DiCaprio has just turned 40. Robbie Collin applauds an actor who is truly ageless.

Leonardo DiCaprio turned 40 a couple of weeks ago, although only in the biological sense. As a performer, he'll remain exactly as old, and as young, as he's been for ever.

For almost as long as he's been acting, DiCaprio has been stranded at a strange no-man's-age - baby-faced and teenage-taut, but with the leopard-ish self-assurance of a man in his 50s or older. It's DiCaprio's agelessness that has shaped his extraordinary career to date, both for good and ill. First it was his trump card, then his Achilles' heel, then latterly, over the past five years or so, it's become his secret weapon.

When you watch DiCaprio as Jordan Belfort, the corrupt stock trader, in Martin Scorsese's The Wolf of Wall Street, you partly see a boy at play, blowing his embezzled millions on drugs, sex and private jets simply because it's what makes him feel good in the moment - but you also see a rational adult, coolly aware of the chaos he's creating, savouring the financial and spiritual carnage rather than boyishly disregarding it.

Belfort is no loveable man-child: he's indisputably an adult, even though he operates with a child's solipsistic single-mindedness. (In other words, he's a psychopath.) That's one of the reasons DiCaprio is perfect in the role: he shows us the man and the child simultaneously, and makes it impossible for us to pick apart the two.

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On the one hand, there's his rosy-cheeked face, grinning a school-photo grin. On the other, there's the rest of him, poised and proud in a bespoke suit, striding across the trading-room floor. In his best roles, DiCaprio has always drawn on that physical tension within himself, to entrance, unnerve and delight.

Compare Jordan Belfort with DiCaprio's breakthrough performance as Jack Dawson in James Cameron's Titanic, and you see the same tricks being deployed to a markedly different effect. As the scruffy artist who wins his tickets to a new life in the New World in a poker game and falls in love with Kate Winslet's blossoming society gal, Jack had to be handsome, but just as crucially, he had to be young.

In Cameron's film, the Titanic is turn-of-the-century Europe in miniature, with strict regulations about whose feet could roam on which decks. But Jack, optimistic and virile, the spirit of the New World incarnate, is an interloper. Peter Chernin, the then-head of 20th Century Fox Film, is said to have preferred Matthew McConaughey or Chris O'Donnell for the role, while Tom Cruise's agent contacted the studio to register his interest.

But Cameron insisted all three were too old, and pursued DiCaprio for months, putting up with his youthful mischief and suggestions for rewrites and dialogue tweaks - which, as a then-relatively unknown 21-year-old working with one of the most powerful directors in the business, he was in no position to demand.

Cameron's instincts and patience were vindicated: DiCaprio played Jack as a loveable underdog, but one with fight and bite as well as bright eyes and a glossy coat. He's young enough to defy the rules of this floating civilisation, but has an old enough head to grasp the significance of doing so. Most importantly, when he called himself king of the world, we believed it.

As an actor, DiCaprio is most consistently fascinating in his own skin, because, since the very start of his career, it's been so hard to square his appearance with the soul we see wriggling underneath. Perhaps that's why he hasn't yet felt the need to give the kind of physically transformative performance pioneered by Robert De Niro in Raging Bull, although with four Academy Award nominations in 20 years yet to yield a win, his agent may be wondering if a dramatic weight gain or workout regime might be what it takes.

At the Oscars earlier this year, DiCaprio's performance in The Wolf Of Wall Street had won him a best actor nomination, but he lost to his Titanic rival McConaughey, who'd dropped more than 19kg to play the campaigning Aids patient Ron Woodroof in Dallas Buyers Club.

McConaughey made a point of hugging DiCaprio as he made his way up to the stage, and DiCaprio received him with an awkward combination of cheek-kiss and shoulder-slap: the body language was that of a high-school wallflower congratulating the new prom king. A collective, maternally inflected sigh of "poor Leo" went up on social media. Even in his tuxedo, as poised as Belfort, as braced for battle as Jack in the first-class dining room, he looked like a kid in need of comfort.

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The face was the same one we saw in 1993, when the world first noticed him. Two great roles came in tandem: as a tortured stepson in Michael Caton-Jones' This Boy's Life and as a teenager with learning difficulties in Lasse Hallstrom's What's Eating Gilbert Grape. By then, DiCaprio was 19 and had passed through the usual child-performer proving grounds of television comedies and toy adverts, plus a couple of potboiler features, one of which went straight to video. But this was a double chance to be noticed alongside serious established talent: De Niro in the former, who had chosen him for the role from among 400 auditionees, and Johnny Depp in the latter, as his older brother.

Both performances are remarkable, but Hallstrom's film was the life-changer. DiCaprio had turned down a role in Disney's Hocus Pocus, a family comedy with every appearance of being a hit-in-waiting, a bold decision that paid off: Hocus Pocus came and went, while Gilbert Grape gave him his first Oscar nomination.

Buoyed by this success, DiCaprio continued chasing odd roles: a young gunsmith in Sam Raimi's western The Quick And The Dead, the lead in the drug-addiction drama The Basketball Diaries, a part initially intended for River Phoenix, and the poet Arthur Rimbaud in Total Eclipse.

Despite his burgeoning heart-throb status, he avoided romance - until 1996, when he was cast in Baz Luhrmann's Romeo + Juliet, and became one half of the most iconic couple in literature. Cinemagoers everywhere swooned, and Romeo + Juliet made almost $150 million worldwide. DiCaprio was a prince among actors, his coronation just around the corner.

But if Titanic was the film that sent him to the top tier of stardom, some time would pass before his boyishness would again work in his favour. What came next was a string of high-profile disappointments: The Beach, The Man in the Iron Mask, Woody Allen's Celebrity, and a film almost nobody saw, an abrasive, black-and-white drama called Don's Plum, directed by a former friend, and the American release of which DiCaprio and his co-star Tobey Maguire blocked with a lawsuit. It took Steven Spielberg and Martin Scorsese to get things back on track. DiCaprio was almost simultaneously cast in two dream projects: as the broad-grinning, teenage con-artist Frank Abagnale in Spielberg's Catch Me If You Can, and as the vengeful orphan Amsterdam Vallon in Scorsese's Gangs Of New York.

Like Titanic's Jack, both characters are self-made spirits, breezily uninterested in sticking to the rules, but each to a different end. Frank craves short-term fun at the expense of middle-aged propriety (embodied by Tom Hanks's FBI agent Carl Hanratty), but Amsterdam hungers for a lasting triumph over his betters, particularly Daniel Day-Lewis' gang leader, Bill Cutting.

His collaboration with Scorsese led to four more: The Aviator, which yielded a second Oscar nomination, The Departed, Shutter Island and The Wolf Of Wall Street. What Scorsese understands is that DiCaprio's lingering boyishness has to be embraced, not hidden or ignored. You spent much of Ridley Scott's Body of Lies (2008) wondering if DiCaprio's CIA case officer was in Iraq on work experience, and in Clint Eastwood's J. Edgar (2010), his face was turned by prosthetics into a rubberised walnut.

DiCaprio doesn't have the kind of gravitas required to blast through layers of makeup. His best recent characters have all hidden their true faces in plain sight: Belfort in The Wolf Of Wall Street, Calvin Candie, the psychotic plantation owner in Quentin Tarantino's Django Unchained, and the title character of The Great Gatsby, his second collaboration with Luhrmann.

This is what gives you hope for DiCaprio's future as an actor. In each of his three most recent films, there is a shot in which he smiles ingratiatingly and raises a glass to the camera. They're welcoming us into the playroom, but they're toasting themselves - each the boy-king of his own little world, standing triumphant on the prow, unworried by the icebergs ahead.