I was 29 when I went back for my Comp Sci degree. Your decision to pursue something that you're more interested in is definately the way to go, especially with the way computer jobs are these days. I took CS because I liked it, and I'm not sorry that I did, but it would be nice to get a job that uses the @#$% degree...
Anyhow, definately hold off on declaring your major for a while. Depending on what work experience you've already got, that's going to play a much larger role in future career choices than your major. A friend of mine, for example, has a BA in Philosophy and and MBA. And he just became a field agent in the FBI. So, outside of specialized professions like CS or accounting, a basic business or liberal arts degree will do you pretty good in a general job search.
Start slowly on the academic load- don't expect to be on top of things just because you're older, or know more, or had some college already. At the university I went to, everyone had to take a math placement exam, and I scored low enough to begin in the remedial algebra class- not even the college algebra one I'd had the first time around, but the one beneath it. I was floored at the amount of stuff I'd forgotten; during the test I kept flashing back to that Simpsons episode where Bart's trying to work out the word problem about the train.
For future job stuff, keep an eye on internships and work study type opportunities. I didn't have much chance to do that sort of thing, since my financial aid was such that I had to work a lot of hours just to make money. And I'm feeling the lack of field experience now that I'm out. Depending on the field or company you're in, you might be able to get some assistance from the company. Also, a lot of school have some sort of "nontraditional" or returning student programs, so look into something like that. It'd be helpful to schedule an appointment with some sort of academic counsellor or advisor and see what all you can access. Find information in a larger university can be tough though; expect to get bounced around a lot between people.