Window of opportunity? According to my doc, you can get them always. I always ask her "when do I need to stop?".....she says as long as they are working for you, you don't need to stop. She went on to tell me that a very elderly lady wanted to continue dancing and needed a "lift", so she got on them.
My workouts are just about everything, upper body, lower body (which I hate)...both free weights, machines, and sometimes bands. I hate going to the gym, but after I'm there I'm glad I push myself to go. It always feels great afterwards. In the beginning I did get a trainer, I wanted to make sure I didn't hurt myself by doing them wrong. Osteoporosis always need weight bearing exercises. Hope this helps.