rock of ages

So, I'm in school again, and besides being a little weird and very exciting, I'm also on the older side of the age spectrum of my class. It's led to more than one variation on this comment when my age comes up: "wow, you don't look that old!"

I understand this sort of statement is supposed to be flattering, and I do take these statements as compliments, but ultimately I find them sort of ridiculous. I mean, I'm 33 and I'm not on meth. I pretty much look my age. I've met a few other people in my program who are also in their 30s, and they look pretty much the same as me, when you account for genetic difference and all.

My question is, where are we getting this idea that 33 is old and that by not looking grizzled, you're doing well for your age? Is this sort of statement just one of those politesse things that people feel they're supposed to say because obviously, as a woman, I must be feeling bad about myself for having attained the ripe old age of 33? Or do people really think 33 typically looks/feels old, and they are genuinely shocked that I seem healthy and energetic and not ancient? Someone help.