At the start, one or two of my characters were as low as 38, while my highers were near 50. Afterwards everyone is at least about 46 (With nothing done outside of Stage 18 itself), but I died a -lot- while doing it because I was really silly. *Shifts eyes*
As long as people aren't too obscenely underleveled, you can probably just jump right in and they'll catch up with the double exp gain. Then again, Stage 18 isn't exactly easy, so maybe not... *Shrugs* Heh :3
edit:What am I saying. No, Satori was about 51 because I NEVER take her out of my party, while my other highers (Alice, Nitori) were more around 48ish. But in general I was closer to the lower forties when I started stage 18. Don't worry about the specifics much though, because a few levelups don't make a big different (Unless you learn an important skill or two on said levels; most characters, however, don't learn much between 40 and 70.)