1A counter for age, used as 〜歳 to express how old a person or animal is. In casual or handwritten contexts the same さい is often written 才 (e.g. 3才), since 歳 is a complex character. Note that 20 is usually read irregularly as はたち (二十歳).
私は今年で二十五歳になります。
I'll turn 25 this year.
「お子さんは何歳ですか。」
How old is your child?
うちの猫はもう15歳だ。
Our cat is already 15 years old.
弟は来月で二十歳になる。
My little brother turns 20 next month.