訓 とし,とせ,よわい ・ 音 サイ,セイ
Sino-Vietnamese: tuế
Han Viet tue, as in tue nguyet (years and months) and van tue (long live). Each passing year adds one to your age, so it is used to count age.
Stroke data: KanjiVG (CC BY-SA 3.0) — kanjivg.tagaini.net
弟は十歳です。 (おとうとはじゅっさいです)
My younger brother is ten years old.