From a Washington Post piece on witty comebacks in the political area comes this (very likely apocryphal) tale:

The perfect put-down has, frankly, little to do with the facts at hand -- just as Reagan could form an effective rebuttal out of his opponent's relative youth, so a young fellow named Alcibiades could demolish an opponent for his age. In a book called "Viva la Repartee," author Mardy Grothe recounts how sometime in the 5th century B.C., Alcibiades debated his uncle, the Greek leader Pericles.

"When I was your age, Alcibiades, I talked just the way you are now talking," Pericles said.

Alcibiades' reply: "If only I had known you, Pericles, when you were at your best."

A quick google has this in a handful of collections of anecdotes ... if it exists, my guess would be in Plutarch's Moralia somewhere.