As someone who started out his IT career at the ripe old age of 27 I would like to think I can offer some advice.
I had always been interested in IT but my first 10 years in gainful employment was actually me being in uniform with the Army, my roles didn't include any IT based work but I kept my finger on the pulse of the IT world (I built my own pc's, read the magazines etc). Just prior to getting out of the Army I gained a Novell CNA certification, with no experience I still managed to leave the Army and get myself an IT job, I started low and worked my way up.
My first IT role was as a HelpDesk agent, answering calls and trying to fix things for the users, I was allowed to flex my IT muscles on some client visits and gained more experience.
I spent my time learning my trade by playing with the technology, there were times when I could have setup a new company infrastructure with the hardware I had at home (infact I can still do that now

) but because of the hands on approach to the technology I got to understand it better. Sure I had my moments.. but learn from them.
One thing I would say to you is that you have to appreciate that you're going to start back at the bottom of the ladder, it's certainly doable if you're ok with that, I have had bosses in the past that have come from the business and moved into the IT area because they wanted a change.. and they took the skills that they already had from the business and applied them to their new roles (just because they hadn't worked in IT before didn't make them useless), with your maturity you have something that an 18 year old school leave doesn't... worldwise experience.
I would perhaps advise against.. and this is just me.. don't waste the time and money on going back to school.. at your age it's perhaps too late to do so, I know that I had considered it but in all fairness it hasn't hindered me at all (I actually left school at 16 with no higher education).
I should add that the roles I generally get involved with now are at the design and deployment level, I design the desktop environments, deployment mechanisms etc, I build, test and handover the server environments for companies.
All of that with no higher education, coming into the industry late but with a few certifications to my name.
SimonD.
The real world is not about exam scores, it's about ability.