This is the 3277th most frequent Japanese word.
幼
"幼" means "young" or "childish."
The word '幼い' (osanai) is used here to refer to 'childhood' or 'something related to early age or youth'.
幼い頃の思い出が蘇る。
Memories of childhood come back.
The word '幼稚' (youchi) in this context means 'immature,' derived from '幼' meaning 'early' or 'young'.
幼稚な言動が目につく。
Immature actions and words are noticeable.
The term '幼友達' (osana tomodachi) combines '幼' (young) and '友達' (friend), meaning a friend from a young age.