I think this discussion has happened before.
I have not used PBworks but am delighted with Wikispaces - free for educators (and they have just announced free wikis for higher education). No age problems because teachers can use the user creator to create users - no email addresses required.
Easy to learn and use.
Lots of Wikispace wikis at my school now - from age 11 to 18.
I've heard that Wikispaces is easier to organize and has better support, but I don't have much first-hand experience.
Katy ScottStretch Your Digital Dollar