About "you don't look old enough to...."

Well, I had a doctor checkup appointment last Thursday, in my first-of-the-year-checkup marathon, and of course had to have a couple of test thingys.
If I had to guess, I'd say the lady who did one of the tests was ten years or so older than me, which would put her in her early 60s. But, maybe I was wrong.
She was very nice and friendly and we chatted as she got everything ready. She was telling me about her training, where she went to school, what she was certified in, etc. I mentioned that my daughter was a rad tech (radiologic technologist, or x-ray tech).
She was quiet a moment, then said, "You don't look old enough to have a daughter that old. And I would not say that if I didn't mean it."
I replied, "Hmmm, she's 29--she'll be 30 in April. I'm pretty old." And laughed, going for the compatriot feeling.
She repeated, "You don't look old enough. I really mean that. I wouldn't say it."
That wasn't the last time during our brief time together she mentioned how I looked--once more before we were done, she repeated that she wouldn't say it if she didn't mean it, but that I looked really young.
And I'd be lying if I didn't say, I hear it a lot. And I continue to be flummoxed as to what to say. Mostly I just say thanks.
But I wonder, why do I look so young to people? I don't look so young to myself.
Sometimes, when I'm feeling a little feisty, I say, "I think I only look young to you now because you didn't know me when I WAS young."
I did know myself, and my image in the mirror, and believe me, my neck was not saggin', nor my eyelids, which, it seems to me, they are doing now. And there's something funny going on around my mouth that I really don't know how people ignore.
But something about me looks youthful to people, and I just puzzle about what it is.
Don't get me wrong--How can you be anything but happy at looking younger than you are? I am happy. I certainly don't want people saying, "Man, Cathy, you look like crap! You look 100 years old! Are you sick!?"
But still. What is it? What is it about my face, or whatever, that strikes people as so much younger than 52?
Is it totally because of the way I look, or the way I carry myself, or some other ephemeral thing? Is it what I talk about--my job as a web site administrator, or that I can help people with computer problems, or music or movies or whatever, that's of this century? Is it my clothes, or my shoes, or my haircut? Or do I just act so stupid people can't imagine I'm a grownup?
I don't know.
It hit me, after leaving the lady at the doctor's office last week, that maybe she wasn't as old as I thought she was, and was struck by the difference, or lack of it, between us?
Maybe I shouldn't analyze it--I'm puzzled, but after all, it's a good thing, and it just is, so...?
How about you? Do you think 50 is the new 30?