This was an issue discussed to me by our family physician when I was having the bulge on my right calf. The bones, particularly of women, become denser and weaker when you reach 40 years. But exercise or workout that includes cycling can be of help. When the muscle is strong, it compensates for the weaker bones. However, there is a limit to that combination. When you reach 60, your bones will become thinner regardless of the muscles that you have. Maybe that's the reason for the higher sales of milk and milk supplements for the calcium needs of the aged.
I think you should be more mindful when you reach 60, but you don't have to stop cycling at that age if you don't have any health issues and you still feel that you can do it. It's best to be on the safe side, though, so that you can enjoy your retirement years even more without any injury.
Before we get too carried away with the fear of bone strength, while it is true that bones do get weaker with age, some people don't have as big of an issue as others due to genetics of course. Everyone on both sides of my family never suffered from bone issues all the way into their 90's and they never worked out! And that includes the women in my family which usually suffer more from bone issues than men. While I agree that weight training would help everyone but for some it would be more helpful than for others. When I retire I do plan on going to a gym but right now I only have time for one or the other so I chose cycling because I enjoy that more than I would going to a gym.
That's the spirit, and I totally feel you. There are seniors at my ride track, that can make young ones to bite the dust. The impact or intensity may change, or maybe the speed, but no one can stop those guys to keep cycling. Some extra calcium and vitamin D may be needed, but the exercise is not going to make them feel worse or even older, on the contrary, that's what keep them active and healthy.The day I stop riding will be the day I stop breathing. I was 69 when I flew over my handlebars, after hitting somebody's home made speed bump in the dark, and landed on my chest. I had a small camera hanging around my neck, and it got between the road and my ribs. I think that's what caused a rib or two to break. Six weeks and I was fine. The loss of skin on my knees and elbows caused the most discomfort.
My doctor advised vitamin D and calcium, so I've been on those for a while. They may have helped....Some extra calcium and vitamin D may be needed, but the exercise is not going to make them feel worse or even older, on the contrary, that's what keep them active and healthy.