If a woman has a colostomy bag placed, how hard would it be to have a baby?

Not much harder. Depending on the reason for the surgery, the woman's fertility should not be affected, although some causes may increase chance of a c section.
If uterus works. You are 34 years old and have a colostomy. You want to know of you can have children. So long as your works. Talk to your doctor about your wish to have a baby. Because you have a colostomy your doctor will most likely refer you to high risk OB at hanneman, temple, einstein or hup. A detailed history will be needed and physical exam. Good luck.