young people have everything going for them physically; they're in the best health they will ever be in; their minds are sharp and clear BUT they lack patience, understanding and wisdom which results in so much wasted effort.Whereas someone who's lived and learn now knows what to do with all that ability of the body but now doesn't have the body to do it with! Another way of putting that saying is; "If I could go back to (whatever age you like) and if I knew then what I know now, life would be wonderful"
It means young people are full of energy and vitality but they fritter away opportunity after opportunity to put their talents to good use, preferring to spend their time playing video games and other pointless pursuits.