In case you haven't heard of this fantastic site, realage.com offers a way to gauge how you are aging by your lifestyle, life circumstances and habits. You fill out a questionnaire divided into various categories (be honest!), and then it spits out your "real age" -- which can be older or younger than your chronological age, depending of course on your behaviors and circumstances. The questionnaire gets pretty specific -- blood pressure, cholesterol numbers, the ingedients in your daily vitamins -- so it helps if possible to have that info handy when you fill it out.
The good news is that you can take control of many factors and make changes to nudge that number downward. The site even offers helpful tips on how to do that.
Facing a birthday in just a couple of weeks, I just retook the test, and seem to be holding my own pretty well -- my Real Age is 6.7 years less than my calendar age. Yay for that!