Is it possible for a young adult to have atrial fibrillation?

Atrial fib (AF): Yes. If no other medical problems and a normal heart as evaluated by an echocardiogram then the usual diagnosis is "lone af" caused by an irritable focus in the heart. Valve disease or weakening of the heart muscle can be a cause. Highly active people (runners) can develop this as well. Treatment varies depending on the frequency of episodes.

Yes: Af is a disease that become more common after age 50. It is unusual, but not especially rare, to have af before this age. Often other conditions can be found that trigger af in young people - like hyperthyroidism, caffeine, alcohol, valve disease, etc. Sometimes af can occur "out of the blue" in young people - usually referred to as "lone af".