There are some things about our youth that we would prefer to forget :)
In fitness terms, youth is mobility. When people see someone who's been on the planet for 80 years still going up & down stairs, carrying their own groceries & headed to the dance hall a couple times a week, they often comment about how young that person seems. It's because they're still moving around independently. It's when we start needing help up off the couch at a family gathering that we start to seem "old" to people. It's that lack of mobility & independence.
The best way to combat that? Keep Moving! Especially your legs! I have a friend who is 85 years young and is still going strong. He said that he realized years ago that most people end up needing a cane or the like just from the muscle loss of age. So he does squats every day.
The squat is the most effective exercise for keeping you "young." You do some sort of squat every time you get up from a chair, a park bench, or the toilet seat. By doing multiple, full range of motion squats, these previous activities should never seem "hard."
So do your squats!
Here's a link to good squat form; keeping the lumbar curve, chest up, knees tracking over the feet, & weight in the heels. In the video, they are performing front squats with weight, but the comparison of good and bad is very good for any squat, even good ol' bodyweight squats.
Quicktime -- "http://media.crossfit.com/cf-video/CrossFit_FrontSquats.mov"
Windows Media -- "http://media.crossfit.com/cf-video/CrossFit_FrontSquats.wmv"