Again, it isn't a matter of age but of the person's health. You can't arbitrarily assign an age to hang up the bike that applies to everyone. For one person it might be 65, for another it might be 90. Nor is a diagnosis of osteoporosis automatically an end to cycling though it should be a factor in making decisions. How severe is the osteoporosis? What kind of cycling do they do and where? How is their balance and coordination? There are so many factors and there is no way that any of us on the internet can make a medical recommendation for any individual. The individual should seek the counsel of a competent physician to determine the potential benefits vs. risks. Ultimately it will be up to that individual to decide what risks they are willing to accept.