65 for most women. It is recommended to get a DEXA scan at age 65 for most women, sometimes earlier if significant risk factors for osteoporosis are present. I think when you say "bone scan" you are referring to the DEXA (also sometimes abbreviated as dxa) scan. See: http://sgoti. Ws/py5wtb.
It depends. Bone scans are used to evaluate for bone infections, recent fractures, and bone cancer, and are not used to check for osteoporosis. Most people never need one. Bone density testing should start when there is a reasonable probability of increased fracture risk that would require treatment. So it depends on risk factors. A common age to start would be 50 for women or 70 for men.