It is 20-24. Paladin is quite good. Up there with other hybrids shamans, druids and priests. Not as good because of the metagame (mass hunters) but quite good.
pretty much every thing else is dead when it comes to getting BGs to pop in less than 30 min.
as for 20-24 you may want to look at what talents most of all going past 20-21 because those two levels get gear scaling whereas a 24 gets no scaling on gear that is only higher out of a BG
some classes it is mandatory to go to 24 because of talents like a shadow priest, other are not
I am attempting to start up a 19 guild on horde & ally, on server Suramar.
If you are into 19 pm me or message me in game btag = Serifaz#1214
NOTE: The bracket is currently really dead, so it will be a while before I can get people in bgs.