There are plenty of artists who put out masterpieces after 30. I just don’t believe they did so consistently. And they certainly no longer captured the zeitgeist.
And one can’t say Bowie was forward looking after 1982. Or Dylan or Cash or Petty or Cohen.
But Bowie’s low came out about a week after he turned 30. But he is a remarkable exception to the general rule.
There may be other great examples. My favorite musician, Andres Calamaro, came out with his best work between 1996-2000 and he was 35-40 years old. He had been successful since the early 80s. Another Argentine , Charly Garcia, comes to mind. In fact when you look international you see lots of great examples such as Gainsbourg, Joaquin Sabina, Caetano Veloso, Jorge Ben. But within the rock and pop idiom being over 30 is almost tantamount to being dead.
Suicide were notable late bloomers.
Bob Dylan with Time out of mind
Leonard Cohen with You want it darker
Bowie
Petty
Cash
Underworld put out all their great stuff after the two founders were over 30. Their Drift box set which only came out a few years ago is incredible.
What is notable about Prince is he cratered. There isn’t even one song that can sit on his greatest hits from his last 10 years.