Since this is a personal thing you can use whatever rules that makes sense. I am 59 and have been riding for 2 1/2 years. My birthday is the end of April so certainly the weather usually cooperates here in the North East. However since any bithday can fall during the work week and I like to ride with my club on the weekends I feel riding my age any time reasonably close to my birthday is fine. Generally my club rides by then start to stretch close to the 60 mile mark anyway so I am safe. I figure within two or three weeks from my birthday is fine and I am guessing I rode 59 miles or over last year 12-15 times anyway.