Here’s some visual evidence:
It’s not that being 74 causing the muscles to decline: the 74 year old triathelete has the same body composition of the 40 year old triathelete. Furthermore, if the the 40 year old triathelete was to be incapacitated for a long period of time, they would start to resemble the 74 year old non-athlete.
This is not to say that our bodies don’t suffer wear and tear as we get older. But we have more control over the aging process than we may think. So get out there and exercise, regardless of how old you are. Just do it. 🙂