Mitsuko Uchida was recently in Cleveland and gave an open rehearsal with the orchestra and then a brief talk afterwards. She said she didn't feel comfortable with her sound at the piano until she was 27, and that was "late, but it is never too late."
On the other hand I saw a round-table discussion video with Charles Rosen, Yefim Bronfman, Emanuel Ax, Leon Fleischer, and Eugene Istomin, oh and Gary Graffmann, and they pretty much agreed that if you don't start at 3 you are screwed. Then one of them, I think Fleischer, made some pseudo-babble about "synapses" "having something to do with it." Frankly just because these people started at age 3 doesn't really mean anything. Fleischer and Graffmann learned the wrong way to play the piano, and were sidelined by injuries. They are now teaching these atrocities to their students, who are starry-eyed with wonder at these so-called celebrities, who are really pianstic criminals who sit on their laurels and feed off their reputations. They would have done better to start at a later age. They cannot speak for those who did not start at 3, and were still successful. So do not let anyone bully you into thinking only those who start early have a chance to play good piano. And do not let anyone who can't play piano well, and safely, tell you how to play piano. Just some advice!
Walter Ramsey