Most people become fatter with aging because they are less active, not because they eat more. Thirty minutes of exercise a day may be enough for heart fitness, but it is not usually enough to lose or maintain weight. Our grandparents doing heavy manual labor were active for 8, 10 or more hours every day. If you have a sedentary job, you need an exercise strategy that includes very vigorous exercise, more time spent in physical activity, or (preferably) both. Researchers from the University of South Carolina showed that the increase in body fat that accompanies aging can be completely prevented with prolonged vigorous exercise ( Medicine & Science in Sports & Exercise, August 2005.) The authors studied 60 thousand male runners and found that the more miles they ran, the thinner they remained. Waist circumference increased with age at all running distances, but the increase diminished in the men who ran further.
This tells us that you must increase your physical activity with age to prevent weight gain. Your chances for success will improve if you find an activity you enjoy enough to do it for several hours at a time, such as dancing, fast walking, rowing or cycling. Then make your sport a regular part of your social life, not just an exercise chore.