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.
He is tall for a 10-year-old.
My grandmother is energetic for an 80-year-old.
Standard kanji spelling using 歳 (age).
Variant using 才, a simplified character sometimes used for age, but less formal than 歳.
Combination of 歳 (さい, 'age') + の (possessive particle) + 割に (わりに, 'considering, in proportion to').