formative
adjective /ˈfɔːmətɪv/
  /ˈfɔːrmətɪv/
[only before noun]- having an important and lasting influence on the development of something or of somebody’s character- the formative years of childhood
- formative influences in the development of the labour movement
 Oxford Collocations DictionaryFormative is used with these nouns:- experience
- influence
- period
- …
 Word Originlate 15th cent.: from Old French formatif, -ive or medieval Latin formativus, from Latin formare ‘to form’, based on Latin forma ‘a mould or form’.