Is it possible to suffer from an abdominal aortic aneurysm at age 18 or you have to be over 50?

AAA age related? Under very rare circumstances a young person could have an aneurysm. Ask your doctor. They can have an ultrasound done if needed.
Yes but unusual. Some patients with connective tissue disease like Marfan syndrome or ehler danlos type 4 cdn develop that early in life. Some unusual aneurysms called inflammatory or mycotic which means infected aneurysms can occur in young patients especially with immune deficiency. Otherwise the most common aneurysms occur in people after there 40-50 yrs of age.
AAA. Inherited disease causing connective tissues anomaly can cause dilatation of main blood vessels very early in life (along with severe joint problems). Otherwise AAA occurs late.