I'm up there too, at 55.
I hadn't thought about regression until I saw this post, so I'm sitting back and reflecting as we speak to see where I think my head is at.
So far I realize a couple different things.
First, I am certain that I am less social at this point than what little bit I used to try to do years ago. But to me this feels more like acceptance as I just don't feel like faking it anymore and I'm happier not trying to interact with people much. When I was younger I kept trying anyway even though I was absolutely no good at it. This caused some measure of distress, embarrassment and regret. Since I don't care anymore, I get to save myself the negative feelings and instead spend my time on the activities I'm good at, which feels much more rewarding. I could be making a huge mistake and painting myself into a corner, but thus far I feel like it's a combination of "I don't think so" and "I don't care anyway". With my family history & genes, at my age I fortunately only have to make it another 20 years (if I'm lucky) and I won't have to worry anymore either way, so there's that.
Second, I'm old enough that my body is on the downhill direction now, and since I know this and can't do much about it, I figure I am better off accepting it, which is much better than feeling dread and anxiety about it. My vision is cloudy and out of focus compared to before (there are procedures I can get to improve on this, but it's not bad enough to need that yet). My metabolism is slow so it's more difficult to stay in shape physically, so I do what I can and accept what I can't. My memory is much worse than it used to be (names, words, etc.) but I continue to get better at understanding complex ideas and mechanisms, so I pull the "senior" card when I forget words & names and declare the improvement of understanding "wisdom". There are a few things about getting old that are good, so I try to embrace those facets. I enjoy the very few long-time friendships I still have which have nuance that's impossible when you're young, and also appreciate how adept I've become at dealing with "life stuff" which was so difficult when I was young.
Anyway, long story longer, you might look at the things that are alarming you from a different angle before assuming that the Asperger's is regressing. Life has a way of churning your foundation loose under foot. The stress of change can certainly push you past some trigger points. I would think that it's much more likely that an aging brain is much more likely to be the source of memory struggles than regression of autism. That's just a guess, but autism is a structural brain thing, a disorder, not a disease. You have the brain you have and it's not like there's an autistic part that's invading your "regular" part. Return of symptoms you had when younger and didn't experience for a long span of time may be defined as autistic, but are probably triggered by changes in your life. Try addressing the situation, or at least your reaction to it.
The first two that come to mind for me are:
1) figure out what may be causing stress and either eliminate that cause or work out ways to calm your reaction to it, or both. This is doable.
2) To address your memory issue, consider about taking up an activity or hobby you enjoy which stimulates your brain in general and challenges your mind. Dive in and geek out on it, including reading. This will improve your memory and your brain's general agility and health. This will take some time to start showing, which is why it's better to start something you enjoy rather than drilling puzzles or flash cards or the like. Something you enjoy can be an effortless permanent new brain exercise rather than a drudgery you won't keep up with anyway. Eventually, you'll just notice that you haven't been forgetting things as much as you were before.