You need to forget one thing.
Since Joymax implemented party mobs the 9 gap thing is a bit changed.
So if you go @ ongs with 14 level difference (char lvl / mob lvl) you still gonna get decent XP/SXP from the pt mobs.
I started grinding on new mobs with a 14lvl difference.
Example:
my lvl 42-44 @ penons 54
...... 50-52 @ PEGs 64
...... 60-62 @ bones/guards and even Generals
The beginning lvls 60-64 @ gens were a bit slow with leveling up but you get 8d drops, so decent lvling up and a bit of economy boost won't hurt you.

Back to the topic,
you can start plvl/farm @ ongs when you hit 20.
The special about ongs is the FAST and BIG spawn, and they have perfect spots where you can position your nubs so they won't die. (ofc. if someone doesn't bring some pt mobs and do a genocide)