Not only will standing on their heads maintain their youth but the lack of tension they feel, the active bodies, and the strength of their muscles will keep them young as well! Our local yoga teacher training program is taught by a woman in her 80s! She must've been standing on her head for years!
However, having said that, I have learned that training to do headstands is a long process. It begins very small, with feet up the wall, neck held still (no looking about), and slowly progresses from there, moving to feet up the wall, hands holding up hips, to feet up the wall, body supported by shoulders...etc...until the student is ready to try taking his/her feet off the wall. It's a nerve-racking moment but even a second of feet off the wall or one foot off the wall is a second one takes pride in.