Can I have a AAA at 20 years old? I have had a mass in my chest just under my left ribs for over a yr and it's grown, but ultrasound showed nothing..

Extremely unlikely. At 20 yrs of age it is really unlikely to have an aaa. The situations where young people have aneurysms are with connective tissue disorders such as marfan's syndrome or ehler danlos syndrome. For a mass under the left rib at your age I'm thinking skin growth or hernia for superficial masses or spleen or bowel pathology for deep masses. Either way a physical exam and ct scan will help.
CT scan. Ask your physician for a chest and abdomen ct scan. This will give you more information regarding this mass.
AAA at 20. Anything is possible but an AAA at age 20 would be exceedingly unusual. A mass not able to be characterized with an ultrasound could be evaluated more thoroughly with a ct scan.