"A youth without fire is followed by an old age without experience" (Charles Caleb Colton).
YOUTH SHOULDN'T BE WASTED LIVING PASSIVELY, WITHOUT INTEREST AND ENGAGEMENT. "Youth," as Robert Louis Stevenson wrote, "is the time to go flashing from one end of the world to the other, both in mind and body." If we let our younger years pass without putting any "fire" into them, we'll suffer the tedium of an old age that lacks the color and the texture it could have had.
If we're now old, what's our attitude toward those who're still young? Does it test our patience to tolerate them, with all their clowning and their pranks and their mischief? If so, hardening of the arteries has probably already set in, emotionally if not physically. We'd do well to maintain gratitude and admiration for the young. Their natural youthfulness should encourage us to maintain that quality as a character trait. Youthfulness is too good a virtue to limit to the young!
Theodor Geisel ("Dr. Seuss") probably did more to remind us of the value of youthfulness than anyone else. "Adults are obsolete children," he always said. And obsolescence is not only unfortunate; it's unnecessary. Like Dr. Seuss, we can make the choice to maintain a youthful outlook long past the point when we lose our physical youth.
Conventional wisdom says that as we get older we have to give up things. No doubt that's true in some ways, but there's another side to that story. It can be argued that the reason we get old is that we give up some things we don't have to give up, like the enthusiasm of youth. There's no holding back the passage of time, obviously, but it is possible to stay youthfully interested in the world around us, and those who've chosen to do so report very gratifying results.
Actually, it all comes down to love. We need to stay in love with our surroundings, our privileges, and our fellow human beings. Continued youthfulness amounts to the determined maintenance of "romanticism" in our lives, the adventurous spirit which wants to taste life deeply in all of its marvelous, intriguing, and sometimes frustrating variety. It means loving life even after we've come to know it fully!
"Age does not protect you from love. But love, to some extent, protects you from age" (Jeanne Moreau).