I dont get how 5-9 year olds usualy learn to play with masters(hired by their parents) waste of childhood IMO...play piano in a cold room , when other kids outside are running around and enjoying their lifes.
I'd say 15-18 is the best age to start any instrument , since then you have some life experience and it makes you more creative.
But some people like Beethoven or Mozart where Pure Geniuses , that happens maybe once in a thousand years.
Know your limit is best advice and know there is room for improvement.