Hi,
searching in some online-dictionaries, the term "formative years" points to the term "adolescence".
https://medical-dictionary.thefreedictionary.com/Formative+yearsPerhaps "adolescence" is a "preferred expression" ( compared to "formative years" ) in the thesauri.
But also "formative years" is quoted, e.g. at the bottom of the above mentioned page, in the "periodicals' archive".
Interesting aspect would be: Do people which are in contact to piano-playing enter adolescence EARLIER than other people?

- I don't think so generally. But for dealing with sometimes complicated stuff ( piano-playing ) one could think that special mental maturity should be existing in children, e.g., who not yet in the common sense may have reached the beginning of adolescence, but in the special area "piano" they might have special gifts.
The problem which occurs, if children too early begin or are drilled too early to play difficult stuff, is in my opinion, that they might be able to play it. But playing is based on experiences, pictures, concepts, feelings one has seen and made up or created from the many things people only can get to know after some longer period of life.
( That's why no 10 years-old-person would win major and earnest competitions, even if he / she was allowed to take part in them ( groups-of-age-restrictions, perhaps ) ) .
If one says to a 5 years old wanting to play "Jeux d'eau":
"Hey, why not imagining a cataclysmal deluge?? Or, perhaps, the prismatic colors of the rainbow, broken in the shattered, watery drops of our innermost desires?? What do you think?"
Then the 5 years old would, unless being very well educated, say: "

" .
( Of course, I must admit, that these words to a 5 years old are only nonsense-ideas, which I have phantasized just now. So pls don't take them seriously, @all. It's only an example.

)
Cordially, 8_octaves.