I believe that if you look at the majority of new/irregular online players, the average falls into the 16-20 group. Then again, I don't know nearly everyone's age and am not so active online myself. For the average SPer you'd have to account those who revisit the game for nostalgia, so I'd say both 21-25 and 26-30 apply to those who grew up with JJ2 when it was new.

Also, someone should vote 1-5 so we can get our Gus back.