formative
adj.adj. describing a time or experience that has a big influence on how your character or mind develops.
adj. relating to a period of development or growth, particularly in early life, that significantly shapes character or personality.
Those formative years at school shaped his entire career.
Many psychologists believe that formative experiences in childhood determine how we handle stress as adults.
The internship proved to be a formative experience for the young architect, exposing her to the practical challenges of urban design that her university courses had only touched upon theoretically.
From Middle English formatyve, formatif (“having the ability to form”), from Old French formatif, formative (modern French formatif), from Medieval Latin formātīvus, from Latin fōrmātus + -īvus (suffix forming adjectives with the sense ‘doing’ or ‘related to doing’). Fōrmātus is the perfect passive participle of fōrmō (“to form, to shape”), from fōrma (“a form, shape”); further etymology uncertain, possibly related to Ancient Greek μορφή (morphḗ, “a form, shape”) (see further at that entry). By surface analysis, form + -ative.
Etymology 2 sense 1 (“thing which causes formation to occur”) is derived from the noun. Etymology 2 sense 2.1 (“language unit”) is borrowed from German Formativ, a noun use of formativ (adjective), from Middle French formatif, from Old French formatif (see etymology 1).
Typically used attributively before nouns like 'years', 'period', or 'experience'.