It comes to all in sport, even the elite; the day when time, body and mind have worked their various effects to the point when the qualities that make a top sportsman or woman are dimmed. What made them world-class no longer applies.
That's the question that can be asked re Tiger Woods, Daniel Vettori and Benji Marshall; divining the answer will be one of the fascinations of this sporting year.
Woods is arguably the most intriguing. The golfing world is bemused at his worst start to a season in his career - a tie for 80th at Torrey Pines and a tie for 41st at Dubai last weekend.
Doesn't sound too bad for an early season warm-up. But Woods was cut at Torrey Pines - a course where he has won eight times previously. His 79 there was one of the worst rounds he has played as a professional.
Woods maintained his game was coming together at the end of the Dubai Desert Classic but he has won twice there and this time did not contend in any way, shape or form. In seven rounds of golf so far, only one has been in the 60s.
It's the first time in his career as a pro that he has started the year without a top 20 placing in either of his first two tournaments. In nine of the 18 years he has been around, he has won at least one of his first two tournaments (and won them both in three of those years).
His familiarity with Torrey Pines and Dubai made them attractive propositions for Woods; top golfers usually perform on a golf course where they have won before.
But his driver is off; his game is off - his 79 at Torrey Pines included a never-seen-before run of seven consecutive holes over par, playing them in nine over. Small wonder many are saying that Tiger is more of a Tabby; that his game is gone.
Tiger Inc is still going strong. Add up the appearance fees he was paid for turning up to Dubai and a skins tournament in India (he didn't win there either) and he trousered something like US$5 million for a week's work. You could say you can't see the Woods for the fees.
But he looks far from the figure who previously projected such bristling superiority over the world's golfers and who said he never turned up to a golf course without being prepared to win.
It looks the end of his quest to beat Jack Nicklaus' record of 18 majors (Woods has 14, his last in 2008).
Maybe, maybe not. Woods is clearly trying a different approach this year. He is tuning up by playing; he's trying a soft launch as opposed to his previous strategy of being ready to win and primed to win from the off. He's never going to say so - the payers of his vast appearance fees are hardly going to thank him (or invite him again) if he tells it like it is: 'I know I won't win here but that's not part of the bigger picture'.
Woods is clearly trying to pace himself this year. Majors are entirely different animals, of course, and many think the 38-year-old Woods will not win another, let alone beat Nicklaus' total.
There are two points to make: first, none of 2013's majors winners and star performers - Adam Scott, Justin Rose, Phil Mickelson and Jason Dufner - has done diddlysquat so far this year.
Let's rely on Nicklaus himself for the second point: Even the greatest golfer yet to stride the fairways was beset by doubts and poor form, twice in his career, when he went for long periods without winning.
Nicklaus almost went bust financially, his businesses occupied his thoughts for a time, his father died, he gained too much weight and he just hit that golfing wall, as all golfers do - even the best.
His second two-year slump came when he was also 38, lasting from 1978 to 1980. In 1979, he didn't win, the only year since 1962 (when he turned pro) he didn't win a tournament.
Nicklaus fixed his swing and his short game and came back back to win three more majors, the last at 46 - significant when you consider that the Tiger critics are pointing out (correctly) that few players win majors after their 35th birthdays.
As for 35-year-old Vettori, it does look like curtains for his career as a player. Hobbling is not a good look; painkillers are not a desirable companion. The Black Caps have made their recent massive improvement without him. The balance of their side looks good; they have depth and options. Spinners Nathan McCullum and Kane Williamson did well, particularly the former, in the recent ODI series against India and the West Indies.
Vettori wants to go to the World Cup next year and his performance in Australia's Big Bash showed he still has the ability. But his Achilles heel is his Achilles heel. A fit Vettori would still be an automatic selection for tests too - but the probability of his body lasting five days through a test, let alone a series, would be a worry though he will still try to get on the plane to the West Indies in May.
It may have to be faced that the team has now moved on; Vettori's absence even a year or two ago would have been the signal for hand-wringing and bemoaning the loss of one of New Zealand's best players in history. But his team have demonstrated what so many fans wanted them to show in the dark days: fight and playing like they care.
They have not only achieved consistency, they have done so by playing textbook cricket shots (stand up, Ross Taylor, Brendon McCullum, Williamson and Martin Guptill) and bowling to a strategy, and with good execution. Vettori would only add to that but, if the time has come to say farewell, at least it is happening when his considerable loss may not be so keenly felt.
Marshall perhaps has the most difficult job, as he is switching codes. He has already had to endure being slipped between two glass slides and put under this country's rugby electron microscope.
His detractors say he is carrying too much weight, his pace has dropped, he has no kicking game, his instinct is to crab sideways, he can't defend ... yada yada yada ...
All of that may be true right now but Marshall is still only 28, with a new challenge in front of him. Players like him have something most don't. He sees a rugby/league field like a pilot sees a heads-up display; things that happen in split-second timing for ordinary old us happen in slo-mo for him. He has an ability to make space for other people; supporting players bust their boilers to get to him. Things can go wrong for him but, when they go right, they often go gloriously right.
Talent worth backing.