"It's not that we have little time, but more that we waste a good deal of it." - Lucius Annaeus Seneca
There is another (non-stoic) quote along similar lines:
"Youth is wasted on the young" - George Bernard Shaw, American Playwright
In other words, youth have no frame of reference other than being young, and when you are young, you have a lot of unencumbered free time. Therefore young people don't appreciate their health, stamina, freedom from demands, and lack of pain. They have not yet experienced the loss of abilities, overwhelming obligations, and the fragility that come with age. They (we!!!) don't appreciate the perks of being young.
