Youth is wondrous yet it is temporary, especially in sport, and Rory McIlroy should remind Lydia Ko of that. Nothing lasts, not pimples nor perfect, fearless days. The Northern Irishman is 24 and the Kiwi 16, both golfing children of a sort, yet as one slumps, the other soars. One has not met expectation yet, the other is being wounded by it.
McIlroy lives the compressed existence of the great athlete: so much done in so little time. At 24, he has won two Majors, reached No.1 and topped money lists on both sides of the Atlantic - which he evidently crossed by walking on water. Now he can't even qualify for the year-end, top 30-players Tour Championship.
McIlroy used to be Lydia. Used to be a kid whose uncorrupted talent made you grin. In his bouncy walk lay his eager ambition, in his unrestrained swing rested his unafraid youth.
Lydia is his demure heir, who calmly strikes the ball as if with a flowing, steel whip. Like McIlroy, she is proof that nothing in sport is as delicious as newness. Every new star is magical possibility awoken again.
Lydia, even if not for much longer, is strangely still an amateur. In a covetous era, her forfeiting of $400,000 for second place at last week's Evian Championship - not to mention over $1 million across two years - makes her appear a sweet anachronism.
Yet her delay in turning professional is demonstration of an uncommon wisdom. It is talent not in a rush. She plays on the LPGA Tour but is not a member of it, immersed in a professional's world yet without its cumbersome responsibilities - she isn't answerable to a sponsor, for instance.
She has played alongside older women, seen them train, seen the effect of failure on them, seen the purity of their skill, and it is the most valuable of apprenticeships. And even as she has won four pro events, golf has been a joy not a job and it is a fine distinction.
Professionals love golf, too, but their self-worth is often determined by results. Their measurement forever - by us and themselves - is through numbers. McIlroy was reminded of this by an Irish paper which noted unsentimentally this week: "Rory McIlroy hits new low: 36 holes, 10 bogeys, 2 double bogeys, 1 treble bogey, 24 shots behind leader."