This one is a toughie. Peoples' personalities can change at any age. I would say you are too young to get married because you are still "Growing Up". However, on the other hand, there have been couples who marry young who have made it for 50+ years and still going...you just never know.
Have you both talked together about everything and how to handle certain situations should they arise? The fact that you are asking this-COULD Be a sign that you yourself are unsure if you are ready to get married right now. Do you two live together? Have you experienced what it is like to wake up in the mornings together and rush off to work/school? You haven't been together for a very long time, you may want to wait a while (like a couple more years) and experience more together as a couple before you DO actually get married.
This is just my suggestion, so I hope you are not offended and that you will think about it.