formative period; formative years
Refers to a developmental stage when something (e.g., a nation, personality, or skill) takes shape. Often used in historical, psychological, or educational contexts.
This country experienced many hardships during its formative period.
Early childhood is important as a formative period for personality.
Compound of 形成 (formation) + 期 (period). The exact historical derivation is uncertain, but the meaning is transparent from its components.