A few years ago I had two senior riders approach me about getting ready to ride a time trial in the Senior Olympics. One was 67 the other was 73. We concentrated on speed work twice a week since they both did LSD (long slow distance) rides everyday. The 73-year-old hit an unbelievable heart rate of 173 on his first 200 yard sprint. In his case and in others I've seen the only limits we have are those we impose upon ourselves. I don't know about an end or a beginning, but 70+ should be a continuation.