I took one of those on-line survey things on this subject and it calculated that I was 47. Since I was 64 when I completed that survey I was pretty happy with 47. The other side of the coin, of course, is that those surveys are pretty worthless!
Big picture, I don't worry about it. I don't have any major physical problems and the minor ones (asthma, very near-sighted, some lower back pain) aren't too tough to deal with. Hope to keep on riding for quite a few more years!
Rick / OCRR