Exercise: the soon the better.

A recent study shown in the Journal of the American Medical Association shows that men and women who are active during their 20s are more likely to prevent substantial weight gain in the future. The key to maintaining a healthy weight, is to "be active and stay active and integrate activity into your everyday lifestyle in a way that you can maintain it."Regular exercise helped keep extra pounds off regardless of a person's initial weight or the amount of calories they typically consumed in a day.

Read more about this study here.