If your biggest fear is getting older, the best thing that can happen to you is getting older. That's right! Like treating a fear of heights with exposure to heights, the universe will also desensitize you to a fear of aging by continuing to apply time + gravity. You will realize that getting older is not only NOT as terrible as you thought, but that it actually it confers untold advantages you couldn't have even imagined when you were busy running around doing cartwheels staying up all night wearing miniskirts.
It's nice to imagine that in a perfect world, you would be the young buck you were at 21, but with all the wisdom you'd have at age 40. The idea here is that you'd have learned a lot of life's important lessons, but have better judgment while still looking great in a miniskirt. But if you were physically 21 but intellectually/emotionally 40, you'd be too cynical and painfully attuned to the risk inherent in everything that you'd never stay up late eating bacon or lay out in the sun without SPF 9000. In other words, what's great about being 21 is not knowing any better so that you're dumb enough to have the kind of stupid fun you can still bounce back from. That can't — and shouldn't — last forever. Have we learned nothing from vampire movies?