If you count the time I spent in the womb I am 70. This is what I know. I cannot train with the same intensity as I did at 40 or 50. I cannot achieve the same heart rate and so go anaerobic sooner. That in itself reduces the intensity that I can achieve. I recover slower and need more rest days so cannot do the volume that I could at 40 or 50. Even 60. But, I still have those moments when it just seems to flow. Those moments make it worth it. Then, there are those rare times when I get to say, "70 year old passing on the left".