Ella Fitzgerald lost some of her range as she got older, but her soul and phrasing got deeper and deeper. The series of duet albums she did with Joe Pass late in her life are exquisite.
Miles Davis was at his wildest and most experimental in his forties. That decade of his life starts with the Miles Smiles band (Herbie Hancock, Wayne Shorter, Ron Carter and Tony Williams) and extends into his electric funk period. The music that he was playing at age 49 on Agharta and Pangaea is the most intense he ever made, though it’s not to everyone’s taste.
I saw Max Roach perform a couple of times when he was in his seventies, and he was tremendous. At one of the shows, he did an encore solo piece on just a hi-hat, and you wouldn’t believe how much music he got out of it.