Artists whose peak period was middle-age

Steve Reich was 39 when Music for 18 Musicians premiered. I think this is common for classical and jazz artists in the contemporary era.

Lucette Bourdin was a visual artist who released her first ambient album in 2005, when she was 51... Then, in following 7 years, she released something like 25 albums, most of which are excellent. Sadly, she died in 2012, but it was an amazing burst of music from someone who had never made (commercial) recordings at all in her first fifty years.