Could all change come patch day, tho. Or at least Dragonflight....
Used to be there were certain levels that were key, like the first talent at 15 could be huge.... now it's talents every level so any level could be the optimal one for your class, spec, or style of play. But every level you go up, so does mob health.
10 was rather difficult on a prot pally on beta in slag mines. That's very specific, but that's all I tested...
At one point i soloed through the first boss of a TBC dungeon on a ret pally at 29, and that should be more difficult. Inflated ilvl and possibly incorrect enemy health may have contributed, I really dont know. At 15 prot pally seemed to do fine. Soloed ramparts which has annoying mobs and many adds if you screw up... slower than live but solid.
I would say look for a talent or ability you feel you cant do without and start there. No need to fall under a specific level.
edit- other than getting access. 15 was also a point of access to more dungeons. Not clear yet how that's going to play out.
edit2- but..... if you're just farming gold then do it at 60 or 70. Not really much use in doing it slow on a 10-20.
