Just turned 60 this month. Began riding a couple of years ago when my knees would not allow running. Bumbled around with a Trek 7500 for awhile then bought the Lemond and put 7000 miles on it the first year (including a solo, unsupported, crosscountry). Am planning on a couple more long distances in the next year or so. Right now I just train 2-3 days a week and play tennis on the other days.
No meds or vitamins, etc. As with all physical exercise, at this age it takes more effort to attain and maintain a given fittness level. When I was training hard for the crosscountry I always rode 5 days in a row and then took two days off. We need recovery time.