It is always a good idea to have one early. This will help you know your normal bone density and it can serve as reference for future examinations. So if you are around 50, it still counts. Bone density problems usually start later on. If you have regular exercise you can reduce the problem. A simple light exercise is a major help. Try walking three times a week and you will probably not have anything to worry about. Now about the tests frequency, it depends on the findings. If there is something you need to monitro, you may have tests throughout the year. It is rarely a surprising condition though. I mean if you are not too thin and not in pain, you are probably fine.