suffix
for one's age; considering one's age
Attaches to a number + 歳 (or 才) to express that something is unexpected or noteworthy given the person's age. Often used when someone seems older or younger than their years, or when an ability or trait is surprising for that age.
彼は10歳の割に背が高い。
He is tall for a 10-year-old.
祖母は80歳の割に元気だ。
My grandmother is energetic for an 80-year-old.
割に is a more general expression meaning 'considering' or 'for', used with various nouns. 歳の割に specifically combines with age.
Combination of 歳 (さい, 'age') + の (possessive particle) + 割に (わりに, 'considering, in proportion to').