Bill Tilden at age 38 I believe. He had won Wimbledon I believe just the year before so he was still superb.
Tilden's record at his best was incredible. He was virtually unbeatable and a true genius of tennis. He invented much of the modern game. Greats like Federer today owe much of their technique to Bill Tilden. I am sure Tilden would have been great in today's game and I'm sure he would have loved today's racquets. You wonder what new theories in tennis he may have figured out in today's modern game.
Edit-Just looked further down in the search results for Tilden and you found the above video a few years ago and posted it on TW.