10-18 they should do the class quests. This gives them all the appropriate extra skills, and puts them on a fast road up. I got to 19 in one night, thanks to that. If there's a gap, I'd suggest the Nautilus quests, as you can do a bunch of them just by killing pigs at the pig farm mini area (they pay alot of EXP too, you can get from lvl15 to the next Aran quest).