There are many reasons to exercise, and there is no good reason not to exercise. However, here is a reason to exercise that you may not have thought of. Exercise now and continue to exercise in order to prevent the functional decline that occurs when you get older.
A large study done at Duke University documented the functional in strength, endurance and balance that occurs from midlife on. To that list I would add mental. Your best weapon to fight the cognitive decline that many experience is exercise. Physical exercise for the body refreshes the brain and mental exercise strengthens it.
Remember the adage "Use it or lose it." This is as true now as it has been for years.