I had my daughter at 19 after falling pregnant at 18.
I would never recommend anyone to do this, but if they were sure it's what they wanted, at least make sure...
*You can drive and have a car already.
*You have moved out of home and have been living under your own roof, with your partner and no housemates, for at least a year, so you know what life is like living away from parents, supporting yourself.
*You have at least a few thousand in savings.
*At least one of you has the kind of job that earns enough for you to both live off while the baby is still young.
*You have done all the "being young and having fun," thing already, and are kinda over it. It still happens after having a baby young, but it can be hard to organise.
Also, as others have said, make sure you are trained in SOMETHING. Something that can help you find work when your children are older. I know you'll think, "We'll be together and happy forever," but so many relationships break down, and having kids can be a real test for them... so if things DO end, you need to make sure that you can take care of yourself and your children. I have been SO LUCKY that I have been able to rely on others... so many people are not that lucky. It makes life hard when you cannot support yourself... so make sure you're qualified at something, and keep updating your skills. It doesn';t have to be uni, but I mean, do a course to work as an admin worker or something... to not do anything is foolish and throwing yourself into the wind and hoping it'll work out your way. Chances are, not everything will go smoothly and you'll need that there to back you up!