Did you ask what results pointed specifically to Down Syndrome? Most all pregnant women are given a blood test that will measure certain things in their blood such as hCG and PAPP-A. If these numbers are abnormally high, then they think it may result in an abnormal pregnancy. There are other tests that can be done that Ellen referred to, such as an amniocentesis test which is where a needle can be inserted into the amniotic sac and then sent to lab for analysis. This will determine if there are birth defects. This test carries the risk of miscarriage. All this being said, your baby could be perfectly normal, because many times doctors are not correct in their evaluations. There are screening tests that are performed that often scare a mother into thinking that her child may be defective. These screening tests are just to tell you how high a risk you MAY have, that doesn't necessarily mean anything, because you may just as well not have anything wrong. But as this your first baby, and if you are healthy, then these risks could be lower. If the doctor is just looking at your age, that should not be the only factor as you are still quite young. But you should go to a OB/GYN that specializes in high-risk pregnancies in case there is a doubt.
The other thing, there is a waiting list for people to adopt babies with Down Syndrome, so someone would love to have your child if it does happen to have a birth defect. But more often than not, you learn to love the child that you have. I love all my kids equally, 1 child with DS, 2 without.
he may be giving you the risk assessment of tri 21 by your age. If you are concerned after having the NT scan and labs you can ask to have an amnio which would give you more info