There is a saying which says "never to late to start" or "better late than never"...however the truth of the matter is that it's best to start when you are very young because it allows you to build a good strong foundation for the future. It's a lot better to start exercising at 13-15 than it is at 40-50...Another thing which needs to be mentioned is the importance of doing weight bearing exercises and resistance/strength traing, especially for cyclists. Cycling is good, but it's not an absolute and cross training is necessary for overall fitness and health.