I sometimes like to scour the early years of the Open Era, because there's a lot of formerly great players from pre-Open Era who played tournaments deep into the 70s. We know know about Laver, Rosewall, Gonzales, as well as guys like Emerson, Gimeno, and Santana playing into the 70s, because they were all still pretty good to great. For example:
Frank Sedgman played in the 1976 AO at 48 years old.
Lew Hoad play 1974 Bournemouth at 40 years old.
Pancho Segura played a tournament as late as 1974, at 52 years old.
Gonzales remains the oldest title winner at 44 years old (1972 Kingston), and the oldest big title equivalent winner at 41 years old, winning 1969 Las Vegas, which was particularly impressive considering that he beat John Newcombe, Ken Rosewall, Stan Smith, and Arthur Ashe, all in or very close to their primes.