While not as stark a difference as Picasso I think Van Gogh is another good example of this, looking at some of his earlier works he was clearly a competent artist, but he really came into his own style later on.
There's some discussion that the weird morphing of his style was caused by undiagnosed mental illness progressing to advanced stages when he got older.
“Visiting an exhibition of children's drawings, Picasso (according to Roland Penrose) said: “When I was their age I could draw like Raphael, but it took me a lifetime to learn to draw like them.””
I saw this quote on a display when I was at the Picasso museum in Malaga last summer