Why not? Airframe may be 22 years old (which isn't very old by airliner standards, by the way), but the interior any many other components are not. The engines may or may not be the originals and the entire aircraft has undergone numerous, mandated, heavy checks in its life. AA's overall average fleet age is 12 years old (per recent company investor presentation), making it the youngest fleet among the other US carriers.

Do you get on the UA forums and ask why they are flying 767's that were delivered in 1991?