It's a technicality. Once you age out you're "not a dependent" for the purposes of educational subsidies/loans.
Getting married has a similar effect, or at least it used to. You could get married, live with one or the other's well-off parents or otherwise be supported by them, but go to school for free or nearly so. Obviously, that's not for everyone :-)
The maximum grant you'll get from the government is about 6k. Anything beyond that is typically the school giving a scholarship or a third party scholarship. And schools typically use a different formula for need aid that doesn't presume you are independent at age 24.
Getting married has a similar effect, or at least it used to. You could get married, live with one or the other's well-off parents or otherwise be supported by them, but go to school for free or nearly so. Obviously, that's not for everyone :-)