Osteoporosis is a naturally occurring phenomenon when bones decrease in density, resulting in weakness and a higher likelihood to break.
Anyone can develop osteoporosis, not just women over 50 years but also men (generally over 70) or anyone who's been bedridden for a long time. The scary part is that bone loss happens without noticable symptoms until a bone breaks spontaneously after a fall that normally would only leave bruises.
Bone loss increases after menopause due to lower levels of estrogen, but may also occur due to smoking, alcohol abuse, undereating, kidney disease, surgical removal of ovaries, taking chemotherapy, SSRI meds and some other prescription drugs.
A change in lifestyle is a good way to slow down osteoporosis: move around purposefully (gardening, grocery shopping, cooking, and moving heavy stuff in general), get enough sunshine for vitamin D, eat nutritionally dense foods with adequate amounts of minerals and vitamins and finally, avoid carbonated drinks with the acidifier phosphoric acid as found in cola.
Since you can't always avoid a fall, it may be a good idea to search for specific courses for elderly people that are mostly offered by teachers of self-defense.