Speaking as an "official" old guy now...
< opinion=personal, projecting=yes >
I think as guys first start getting on in years (gay years, that is) -- i.e. the hair starts going gray or thinning or receding, and the body fat gets tougher to keep off, and the joints start acting up, etc. (this could happen to guys anywhere from late 30s to late 50s, depending on the genetic cards you've been dealt, and how well you've played them), they are only getting used to the idea of aging themselves, so I classify them as "age sensitive". An "age sensitive" guy is probably not going to react well to the "for your age" modifier, cause it's just another piece of evidence he can't ignore, regardless of how well meaning the compliment -- and frankly, they are all well meaning.
However, once a guy gets past the "age sensitive" years, and starts to own his age and his body (with all of its new "quirks"), then I find one is no longer all that sensitive to any new signs of aging, including the "for your age" modifier. It's all very familiar, and it's not any big fucking deal anymore. We're just happy for what we have, and value it more and more every day.
I'm happy to say I exited the "age sensitive" years a few years back, so I have no problem with a young guy saying I look good for my age. I take it, gratefully, as the compliment it was meant to be.
< /opinion >
PS I want to give a shout out to all the young guys who've been so freaking brilliant to me, in so many ways, many of them here on RJ, to me in my "old age." I only wish I could reincarnate as a 20 something when you're all in your 50s and return the love.
I'll guess I just have to be more appreciative of you all now, while I'm still here