See the link I gave in post #7; there’s a serious answer in there. In short, they buy lists of kids with addresses and ages from any source they can find, including lists of kids who wanted free ice cream.
*** Ponder
Thanks for the link. I saw the link before, but I guess I thought it was a joke and didn’t click. Sorry. 
I know that many dopers are from outside the US, so I wanted to point out two things. One, only males are required to register. Failure to do so not only makes you ineligible for government employment and benefits, but you can be sentenced to up to five years in prison. Two, we have not had a draft or any other form of compulsory military service since 1973.
<imho>Why are we spending 25 million dollars each year (for the past thirty years) to register young man on the off chance we might decide to reinstate the draft? </imho>
I think, as I mentioned above, Drivers Licenses are probably a good part of it. When I went in for Jury duty the said that was a large part of how they got the pool for potential jurors. I can’t remember if I had to supply my SSN when I got my DL but it seems pretty reasonable that it was on the form. Voila, government administrated list with your name, address, age and SSN. Since you are supposed to update your address on your license and have to renew it every few years it seems a pretty convenient way to track you.
I know not everyone has a license but I would guess more people have a Drivers License or State ID than don’t.