It's really random, because I've never been to Mexico.
Still: I ran across this link over at Randy McDonald's Livejournal. It suggests that Mexico, having gone through the demographic transition, might be headed for a Spanish or Italian-style demographic collapse. That led me to google up the time series on Mexico's Total Fertility Rate (births per woman):
Well, two things jump out there. One is, that's a steady, sustained decline. (It actually extends pretty smoothly back into the 1990s and beyond. I just didn't feel like typing in another ten years of numbers.) The other is, it's a pretty slow decline. TFR is falling by about one-fortieth of a kid per woman per year. (N.B., I know that TFR is very far from the whole story. Let's put discussion of birth tempo and such aside for the nonce.)
Let's assume for argument's sake that it continues. What then?
Well, Mexico is not going to turn into Spain or Italy any time soon. Replacement TFR is around 2.07. Mexico should hit that around 2020. But demographic inertia -- oh, just google it -- will keep Mexico's population growing, albeit slowly, for another generation after that. And it won't approach Spain or Italy's TFRs (around 1.4) until 2050 or so. Which is quite a ways off.
Also, I'm inclined to think that Mexico will stabilize at a point well above 1.4 kids per woman. Call it a hunch. Some of the cultural factors that operate in Spain and Italy -- like adult children living with their parents far into their twenties and even thirties -- don't seem so strong in Mexico, after correcting for income disparities. And, well, Mexico is in North America, and North America just seems to follow a slightly different path than Europe.
Anyway. One thing that comes to mind that I don't think has been much discussed: say Mexico does fall below replacement fertility after 2020. And 2020s then see Mexico's population growth slow down (though not quite stop), and its population begin aging fairly quickly.
What impact on the US? Immigration is currently driving US population growth, and the single biggest chunk of that is from Mexico. And the use of Mexico as a source of cheap and mostly young labor... well, that would suddenly start becoming more difficult. And then there's the whole intertangled immigration /illegals / amnesty / bracero debate. Demography isn't the only driver of immigration, to be sure -- people are always going to move away from poor countries to rich ones -- but it's a big factor. Goodness only knows what this stuff will look like in the 2020s, but how will the whole debate change when the pressure on the borders starts to ease?
Eh, it's all very hypothetical at this point. Still: a Mexico with below-replacement TFR may be just a decade away. Go figure.