I think you should get a physical and complete workup by a medical professional. Different body chemistry will show up on people at different stages of their life. If your doctor says you are healthy I would let nature take it's course and you will gain weight with age, as most of us do. If you force the issue you may pay down the road when your metabolism slows down. Just because you think or somebody else thinks you are too thin does not make it so. Get advice from your doctor.