Woods, who will be 40 in December, has been struggling for form heading into next week's British Open at St Andrews, where he won the Claret Jug by eight shots in 2000 and only a slightly less commanding five strokes in 2005.

Those days of dominance are now long gone, however, and Cowen, who has tutored some of Europe's best golfers including Henrik Stenson and Lee Westwood, thinks it is too late for the American to ever get them back.

"If there is one important thing that makes any sportsperson look ordinary and that is age," Cowen told Reuters at the Scottish Open.

"You can't do anything about a person's age. That invincibility diminishes with age and it has to. But then once you lose that invincibility how do you get it back? "In all my years of coaching, I have not seen one player get back that invincibility."

Cowen said that the younger players no longer feared Woods and that the generational shift in golf is taking place more quickly than it did when the turnover at the top of the game happened every two decades or so.

The Rotherham-based Englishman said it should also not be underestimated how much mental toll Woods's long period as the world's best player took on him.

"Everyone ages differently and I am not talking physically but mentally," he added. "People don't realise that you are in the mix in every single tournament, and years back Tiger was in the mix in every tournament he contested, that mentally is draining.

"I can see when Henrik is in the mix because at the end of the tournament he is absolutely, totally mentally drained and exhausted, and he has not won near as many tournaments as Tiger."

Woods shot his worst round as a professional last month, an 85 at the Memorial tournament, and he also missed the cut at the U.S. Open. Cowen, however, believes the American has earned the right to a bit of respect for his achievements and the golfing world should be grateful, not critical.

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