A man who does not exert himself in his youth will be sorry he didn’t when he grows old.

解释 ( paraphrase )
年轻力壮的时候不努力干出一番事业，年纪老了再伤悲后悔已经没有用了。
这条谚语告诉我们：不要虚度年华。趁年轻的时候抓紧学习，努力工作，干出一番事业，到老了才不至于感到后悔。

This proverb points out that if a person fritters away his youth in idleness, he will regret it when he grows old, but by then it will be too late.

徒：徒然；白白的；毫无用处地。
       In vain.

造句 ( example )
老师经常告诫我们：“少壮不努力，老大徒伤悲”，只有现在好好学习，长大后才能做一个有用的人。