"The youth is wasted on the young"
It is by nature that humans are never satisfied with what we already have. The 'old' frequently find ways to look, feel and be perceived as young while at the same time, the 'young' are always obsessed about wanting to be older. To say that youth waste their youth wouldn't be fair as youth is frivolous by nature and I don't see why it is improper for most to spend it in a manner befitting its nature.
For some, youth never truly leaves them.
Unfortunately, when adulthood finally comes knocking on the front door, it becomes apparent that it falls considerably short of our expectations in many ways. Confronted we are by the fact of reality: freedom and independence comes with a hefty price tag attached. Responsibilities become unsheltered and sheltering responsibilities from the innocent has become inevitable.
As it slowly begins to dawn on us that our existence here is a finite thing, we learn to truly slow down and appreciate all of the things that frivolous youth allowed us to take for granted. We learn to appreciate the simple joys, like sunrise, and milk before bed; we learn to laugh more; we learn to love deeper; we learn to forgive rather than fight.
If that is part of old age, I sure as heck would like to be with it! But I don't mind waiting another 40 years :)