They who would be young when they are old must be old when they are young.
Meaning: "Young people should take the opportunity to do all the things they will be unable to do when they're older." Source for meaning: Martin H. Manser (2007). The Facts on File Dictionary of Proverbs. Infobase Publishing. p. 318. Strauss, Emanuel (1994). "1605". Dictionary of European proverbs. II. Routledge. p. 1151.