乳幼児[nyuuyouji] infants
幼い[osanai] very young
幼馴染[osananajimi] childhood friend
幼魚[yougyo] fry (young fish)
幼形進化[youkeishinka] paedomorphosis

幼子[osanago] infant
幼児[youji] infant
幼児ポルノ[youjiporuno] child pornography
幼児語[youjigo] baby talk (like choo-choo for train, etc.)
幼時[youji] childhood

加齢[karei] adding to one's years
学齢[gakurei] school age
月齢[getsurei] age of the moon
高齢[kourei] advanced (old) age
弱齢[jakurei] youth

寿齢[jurei] long life
樹齢[jurei] age of a tree
就学年齢[shuugakunenrei] school age
適齢期[tekireiki] marriageable age
年齢差[nenreisa] age difference