Can an 18 old girl who has had chemotherapy still get pregnant later in life?

Yes, usually. If your menstrual cycle has come back, you can get pregnant! however, many women who have had chemotherapy in their adolescent or early adult years will go into menopause early. If you haven't had chemotherapy yet, it's a good idea to talk to your doctor about ways to preserve your fertility. They can now harvest eggs from your ovaries and store them for later use.
Yes. If you are having regular menstrual periods following chemotherapy, you can get pregnant and have a normal, healthy baby. Any problems the drugs can cause a fetus go away after completion of treatment. Many cancer survivors have gone on to have health babies.