Steve Nash is 40 years old and is trying to prove to himself and the world that he can still cut it in what is probably the most gruelling sports league in the world. He likens the mounting uncertainty about the physical gifts he once accepted as a natural part of himself as a "dark presence" and explores it in a short film called The Finish Line , which anyone who is considering winding down their sporting life should watch.

In doing so, he is debating the question that all sports people at whatever level are forced to ask themselves: when I can no longer do this?

Brian O’Driscoll, arguably Ireland’s greatest ever sportsperson, has decided that the answer comes at the end of this season. O’Driscoll is 35 and has spent all of his adult life taking a physical pounding during a period when the body shape and mass of rugby players has undergone a cartoonish transformation. When O’Driscoll hurls himself into the path of an oncoming juggernaut like George North or Jamie Roberts now, you begin to wince. The man has taken enough poundings for club and country. Rugby has become an absolute war of attrition and there is only so much any physique can take. O’Driscoll is doing the smart thing.

But in general, there is an expectation – and obligation – that sports stars exit the stage earlier and earlier. Any elite GAA star that remains on the stage past the age of 35 is regarded as somewhere between exceptional and strange.

Tony Brown, the Waterford hurler, is treated as a kind of human wonder because he has the desire – and the athleticism– to keep on hurling at the highest level at the age of 40. It is true that not every ball player has the kind of flexible athleticism and lightness of frame which enables Brown to live with the speed and suppleness of hurlers 15 and 20 years younger than him. But many do and decide to go anyway.

Nash is one of the most fascinating basketball players in the NBA, mainly because nothing about his athletic potential suggested he should be earning $10m per year playing point guard for the Los Angeles Lakers, probably the glitziest team name in sport.

Nash has one of the least typical backgrounds in the NBA: born in Johannesburg to Welsh and English parents, grew up in Saskatchewan where he played soccer and ice hockey and only took up basketball at the age of 13, when most potential NBA prospects are already known by those in the business. But Nash escaped the scouting system, was not recruited by any of the major colleges and was still a relatively obscure name when he was drafted by the Phoenix Suns in 1996. In the 18 years since, he has earned the right to be classed among the very best point guards ever to play the game and he plays it with a style and imagination that is entirely his own.

Uncanny nimbleness
He is of average height, slim build and not particularly quick but possesses an uncanny nimbleness that enables him to exploit the tiny windows of opportunity left by the behemoths who guard NBA baskets. Often, he makes the bigger men look ponderous in their movements because Nash himself moves like liquid and has this extraordinary facility to deliver no-look, oblique passes which deceive not just opposing teams but the crowd in the arena and the millions of NBA fans watching on televisions around the world. In short, Nash is a delight to watch. And he is his own man.

During the World Cup in South Africa, a number of us were pleased to see a chirpy Kevin Keegan emerge from a pub in the Sandton shopping plaza, a sort of above-ground plaza with hotels and restaurants overseen by security guards. Fans from various countries would gather to ‘entertain’ visitors with impromptu concerts from their own fair land and KK, as ever, summed up the situation best when he shouted, with no little glee: “Oh look! The Mexicans are havin’ a sing-off against the Brazilians.”

Unfortunately, there were no other 1980s football legends in the boozer but there was a guy who looked incredibly similar to Steve Nash – but surely not tall enough to be the guy who had spent 15 years floating left-handed shots off the glass despite the best efforts of 6ft 11ins ballers who should, by rights, have chewed him up. But it was Nash, having a beer and watching a game on the box and, it turned out, acting the hobo on his way around South Africa for a series of indie documentaries which aired on CBS.

He has set up numerous charitable foundations, actually expresses an opinion – he was one of the few NBA stars to speak out against the Iraq invasion before it took place – and could easily move into media work if and when he retired. So why, after breaking his leg last season, did he decide to try and fight his way back?

Cynical reason
The obvious if cynical reason is that Nash can pick up a cool 10 million from the Lakers just by hanging around next year. But he already has immense wealth from the game and could inevitably find different ways to make money when he leaves.

No, Nash is at the point where although he is operating in a remorseless sports business in which players are commodities, he just wants to continue playing the game he loves. On one level, Nash to all intents inhabits a different world than Tony Brown and other GAA stars (although he has never played in front of an 80,000 crowd). But on another level, he is precisely the same as them: a human who worked ferociously hard on developing his skills set in his chosen sport and loathe to lay down those tools before he absolutely has to. And it is a delicate one. Yes, there is something to be celebrated in Ryan Giggs’ continuing presence in Manchester United. But then you remember Giggs as the lithe, curly-headed kid with the preternatural balance and there is the nagging fear that he might end up as the old guy at the rave.

But when is too old? If Michael Jordan was only half joking when he announced in his hall of fame speech that he might make his third comeback at the age of 50, he made people ask the question: could he do it? Footage of him dunking the ball after he had turned the half century makes it clear that Jordan still has game.

The most that Nash can hope to play among the very best in the world is for another two years. You can see why in the few seconds of slow motion footage of his comeback game for the Lakers: just the smooth grace and the way he mesmerises three Timberwolves players with the ball just as they think they have him trapped in the corner and then freezes them with an effortless yo-yoing bounce pass to a free team-mate. Nash can do that better than almost anyone on earth. That must be a pretty magical feeling.