Fun fact: only three players have won pro tournaments in the Open Era after turning 38 years old: Pancho Gonzales (5 times), Ken Rosewall (12 times), and Roger Federer (once).
Pancho, of course, is known for being the oldest tournament champion at 43 years and almost 9 months: Des Moines (ATP 250 equivalent) in 1972. But even more impressive was his title at Los Angeles Masters level tournament just a four months earlier, including defeating a 19-year old Jimmy Connors in the final.
Among active players, the 20 oldest title champions are held by three players: Federer, Ivo Karlovic, and Feliciano Lopez.
If Roger wins another Grand Slam, he will surpass Ken Rosewall as the oldest Slam title winner. Rosewall was 37 years, and just shy of 2 months when he won his last Slam, the 1972 Australian Open. Federer was 36 yrs, 5mo when he won the 2018 Australian Open.