Would you want your child getting pregnant at 13? Knowing that it may kill her or that people would look down on her for the rest of her life? Wait 5 years and if you are still with this guy, THEN try! Right now you would damage your body so bad...not to mention the baby could end up really sick or even die because you are so young. Your body is still maturing itself. You can't even get a job to support a child at 13.
How would you provide healthcare for you and your infant?
What kind of life could you provide for your child?
How would you provide for your child when you are still a child yourself?
What would your parents think?
Would it ruin your relationship with them?
Would you drop out of school?
You can NEVER be sure that the father will stay with you. More than likely it would all become too overwhelming for him, and he'd split, leaving you a single mom.
Taking care of other peoples kids is WAYYY different than taking care of one 24/7 for 18 years...
Babies are expensive and they aren't all fun and games. They aren't babies forever. They grow up then they cost even more. Do you really want a toddler/pre-schooler when you are supposed to be in high school?
Two common problems of babies born to young teens are premature birth and low birth weight. These create a many problems on their own including brain damage, physical disabilities, etc. The long hospital stay and health problems for these babies puts tons of stress on the teen.
If you were mature enough to have a baby, you would know you need to wait, and you'd have the self control to wait.