It would be extremely rare for someone your age to have a AAA. Genetics, lifestyle choices like smoking, and physical trauma all play factors. Eastern European backround people tend to get them the most. I work in this area and have not seen anyone under 45 with a AAA> Pulsating can be seen in very petite or thin people. However, if still concerned, one can get an ultrasound which is easy and quick. At your age, I would guess the only way this could happen is if you had some major physical trauma. ie major car accident. Good Luck!
If you're worried you're having an abdominal aortic aneurysm, you need to go to the ER. If you do have one and it ruptures, you're pretty much dead. Hopefully it's just your heartbeat in your abdomen (which is pretty strong in me), but better safe than sorry.