Happy B'day. I am six years and not quite one month younger than you. 6-7 years ago, I found that I seemed to be losing strength and stamina as I approached 50. My joints bothered me and it was getting progressively harder to push big boats in heavy going. As a result, I began doing yoga, and paying more attention to my muscle tone, and my weight. That combination has made a really big difference to my ability to aggressively sail short-handed in a wide range of conditions. Staying in shape doesn't come as easy as it did when I was younger, but it hasn't been all that hard either.
For what it is worth, I am sure that genes play a big role as well. Dad is turning 80 in a few weeks and he is still a great sailor. He threatens to one day to buy a motorsailor or a trawler when he gets old.