若さ

プログレッシブ和英中辞典(第4版)

わかさ【若さ】

youth; youthfulness

あなたの若さの秘訣は何ですか
How have you managed to stay [keep yourself] so young?／What is the secret of your youth?

彼は25歳の若さで死んだ
He died at the early age of twenty-five.

彼は60歳だが40代の若さだ
At sixty, he still retains the vigor [energy] of a man in his forties.

出典　小学館「プログレッシブ和英中辞典(第4版)」