A formative period of time or experience is one that has an important and lasting influence on a person's character and attitudes. She was born in Barbados but spent her formative years in east London. having an important influence on the way someone or something develops formative years/period/stages etc (=the period when someone's character develops)
minimal language unit that has a syntactic (or morphological) function forming or capable of forming or molding; "a formative influence"; "a formative experience"