This question has occurred to me as well...but ultimately, I decided not to worry about it. Life happens when it happens.
Personally, I'm not even sure if I want kids. If I meet someone, fall in love, and decide to start a family at some point...that's how things were meant to turn out I guess. I do think family is important, and it might be good to put my pesky nurturing/maternal instincts to good use. But if not, then I can take comfort in the fact that I'm not contributing to our plant's overpopulation and depleting natural sources. Plus, the women in my family tend to get morbidly obese after having kids. There is also the family problem of addiction and crippling depression (makes for some exciting teen years, though!).
But seriously...don't focus on that; you have plenty of time to make decisions about kids and all that. Focus on what is before you right now. Life has a way of falling into place.