The Journal of American Medical Association found that if you exercise 2 1/2 hours every week, you could be 13 lbs lighter in middle age than someone who doesn't work out.
Just keep in mind that a little weight gain can be a normal (and dare I say healthy) part of aging. Of course when I say this I am referring to people who have always lived their life at a healthy and fit body weight for their height, frame, and age. Even for fit people, body composition shifts with age making it ever-so -important to focus on resistance training along with cardiovascular exercise.
My (#$-year-old) mom looks amazing, but doesn't weigh what she did at age 30. In fact the other day she explained to me that she can't go under a certain weight or it makes her face look older. There is wisdom in that!
She still gets mistaken for much younger almost on a daily basis. Can you believe she has a 34-year-old? One of these days I'll share her aging secrets.
In the meantime, set aside at least 2 1/2 hours each week for cardiovascular exercise!