It's most active yeah, but not for "good" reasons. People play on trial accounts (no longer 10 day, infinite with restrictions) they are stuck at 20, the bracket is 20-24, that's why it's active. There are also players 20-24 that are P2P (Pay to Play, normal players, subscribers) (F2Ps usually hate them). With WoD brackets are going back to 10 levels (apparently) so it'll be 20-29, this means some 24s might level to 29, or stay at that level for scaling or such. But it still should be the most active bracket due to F2Ps. However play it now or play it then, it's dogshit.
Give 19s a go, if you like it, awesome, if you don't, hang on for WoD, looking pretty good so far.