I actually wonder the same thing sometimes -- I'm 30, went to MIT from age 16-18 but dropped out due to lack of money, went to found a few startups, and am now doing defense contracting (for another few months).

When I get back, I am trying to decide what to do -- mainly a startup full time, or a startup/consulting part time plus going back to school (presumably either MIT or UW, maybe Stanford). I'd probably do aero engineering at UW, or nuke or biomed or mech eng at MIT, and then would do a startup in those fields.

The main benefits I see for a degree at 30, when one is already pretty well established in the work force, are access to grad school, and foreign visa apps. While I could certainly learn useful CS in an academic program at this point, I think I'm able to learn on my own about as efficiently by now -- however, that's less true for a new field.

From a purely financial standpoint it is probably a bad idea; 2-4 years of lost earnings, plus direct costs.

At 18, I would almost definitely do college (actually, I'd probably take 1-2 years off and THEN do school, and try to do a program which had a lot of coop/work experience).

If nothing else, avoiding going into the job market for 2010-2014 is worth it.