eddy, Manly gave you great advice, but if I can add to it: There is a tension between (and I hate saying this) being old enough to seem capable and having experience under your belt. That is to say, given the chance to hire someone who is 28 vs. someone who is 24, some employers may go with the older one in the hopes that he's more mature. This isn't always the case (especially for people 50+, who get screwed out of work sometimes because people think they're too old), but I certainly experienced it myself. Even now, at 30, I get blowback from people saying "how old are you, 23?" suggesting that I'm not old enough to "get it." Part of me is happy that I look youthful, but a greater part of me is pissed off that even now people are treating me like that. The last time it happened to me was Wednesday, actually. It's unfair, no matter what age you are. But it happens.
Meanwhile, if you're any age and have no experience that will hurt you - regardless of your credentials, in my experience. It looks worse and worse the older you get, though I suspect that's changing with so many people changing careers later in life these days, like Industrialsize's experience shows.
So, to add to what others have said, I would encourage you that any age is fine to start your career - just make sure you've done as much volunteering, networking, and odd-jobing in the field as you can. That's how you meet the people who will want to hire you into your career - the people who will know, no matter how old or young you are, what you look like, what your gender is (or any of the other things that can be obstacles to getting a job), how capable, smart, and good of a co-worker you are. Because
that will get you hired over anything else. (As I'm sure others can attest, most jobs aren't advertised. Most jobs go to people who know people.) Also, that gives you the real-world experience to decide if the career you think you want is the one you really want. I know I did a four-year degree at uni (actually, I did five years' worth of credits before my graduate degree started) and it gave me just enough time to research all the different paths there are within my chosen career and then settle on the one that fit best with me.
Also, welcome to the forums, mate - and good luck with uni. It's a shitload of fun, if you let it be