young once
A phrase emphasizing that someone will not be young forever. Typically used in the phrase "you're only young once." You need to go skydiving with us. Come on, you're only young once—it's time for an adventure!
you're only young once
You will not stay young forever, so you should take risks and try new things now while you are free and able. You need to go skydiving with us. Come on, you're only young once—it's time for an adventure! I decided to sell my things and buy a one-way ticket to India. You're only young once, after all.