Is it common for teenagers to have atrial fib?

No: If a teenager has atrial fibrillation, damage to the heart needs to be excluded.

No.: 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".