(one's) age
「歯」 has other readings with different meanings.
tooth; teeth
tooth
歯を重ねるごとに体力が落ちる。
With each passing year, my stamina declines.
まだ若い歯なのに無理しないで。
You're still young, so don't push yourself.
歯は七十を超えたが毎日元気に散歩を続けている。
He's over seventy but keeps walking energetically every day.