Whenever any athlete gets old, you usually realize it all at once, and it becomes a disturbing reminder of how quickly time passes. This happens with superstars in any sport. Did you know Tom Brady is 34 years-old? Whether you were 14 when you first watched him or 24, that probably makes you feel old.
It reminds me of this essay from The Awl, where Shaq wasn't just a retiring Hall-of-Famer last year, but a "THIRTY-NINE YEAR-OLD REMINDER OF DEATH.