for ‧ ma ‧ tive /ˈfɔːmətɪv $ ˈfɔːr-/ BrE AmE adjective [only before noun]
having an important influence on the way someone or something develops
formative years/period/stages etc (=the period when someone’s character develops)
He exposed his children to music throughout their formative years.
formative influence/effect etc
International politics were a formative influence on the party.