Rest assured, you will hear the 'just wait and you'll probably change your mind' for a long long time. Both my girlfriend and myself have been hearing it for a good 10 years now, ever since we've realized that we don't want kids, ever. We haven't been together for 10 years, we had both decided that before met, and we actually talked about it before getting into a serious relationship since it would've been a dealbreaker for me (and, as it turned out, for her as well).
That aside, 23 is, as others have said, a bit too young to start talking about a biological clock ticking. From what I've read, and seen in my group of friends, that generally starts at about 28-30. 23 is actually considered pretty young to have a child amongst my friends and family, though that probably depends where on the 'social ladder' your friends and family is situated (I know it sounds horrible, but nonetheless it's mostly true, here in the Netherlands anyway).
It's just a choice you have to make, and once people realize your mind is made up they are usually fine with it as well, some (specially parents) might take a while to get used to it though.
Also don't forget, if you're not sure you want them, and regret it when you have them for whatever reason (peer pressure comes to mind first and foremost), it's not just your life that gets screwed up but also that of the child, who hasn't had a choice in it at all! Personally I would say that if you're in doubt everyone will be better off that you don't have kids. The world doesn't need more people to survive (quite the contrary) and at least here in Europe kids aren't essential when you get older, that's what you've been paying taxes and saving money for all your life.
Having no children opens up lots of possibilities, and we're loving it! We both have a decent job, nothing amazing but a bit above average wages. We go on a far away vacation (SE-Asia / Africe / S-America) at least once/year, usually an in-Europe vacation as well. We go out for dinner at least once a week, we can eat and drink what we want, buy books /DVD's and whatnot whenever we want.
Compared to our friends with kids the things we can afford are just amazing, most people don't seem to realize how much time and money you can save by having no children