Believe it or not, I actually have asked an AMP girl, on many occasions, where she lived (as well as where she 'was from'). This was usually in response to her asking the same question of me. In some cases, the girl drove a fair distance (say, from Rockville to work in a Virginia AMP), while in other cases the girl lived close by (relatively speaking). And, in some few cases that I've encountered (your findings are apparently different), the girl was actually living at the AMP. In all cases, though, I never got the impression that any of the girls owned a home or apartment, but instead were renting for longer or shorter periods of time, or possibly even renting a room with some other non-AMP girls who were either aware or unaware of the AMP girl's occupation. There's a lot of options in regard to where an AMP girl lives.
I still maintain that lower rent prices, being generally cheaper in the outer metro regions, explains the preponderence of younger girls at AMPs in those regions. It may not fully explain it, but I'd wager it explains it in part. As for the propensity for AMPs closer in to have older gals, rent again plays a large part in that, IMO. But, the (presumed) reliability of older gals might also be a factor. In any respect, I entirely reject the simplistic notion that AMPs close to DC have older gals simply because we've somehow "given up" demanding "better" and that we've capitulated to some dire plot to foist older women on an area that has one of the highest per capita incomes and, thus, disposable liesure budgets in the country. But, until we conduct a scientific survey and control for various factors, we're just tossing around opinions. But it is, like Lex said, an interesting point of inquiry.
R::R