Just brisk walking and swinging your arms is wonderful. A little weight lifting to keep those muscles stressing your bones to be strong is also wonderful. Just keep your body moving. Drink plenty of water and exercise. You don't necessarily need to gain weight.
Bone scans are another test.
I am not sure about age, but I would guess around 35. I hate exercising. I've managed to avoid it for 43 years. Get some good shoes. Put earphones in and march away to the Canadian Brass, my personal favorite for exercise.