I guess "When You Were Young" is a sort of ode to youth or maybe more of a lament of it's demise and of the people who can't wait to lose it.
You know those who just want to be grown up, that lose there sense of fun and wonder. Life becomes a weight they love to carry around, everything is so serious.
What the song wants to do is point them back to when there was magic, hope and that innocence of youth. Innocence is something we should always treasure, because this life does tend to drain it away.