@Alconja There's no strict requirement, as in you won't get booted out for inactivity unless you, say, stop visting the site / handling flags for several weeks without previous notice. You should be able to check in at least one or two times a day, though, to check the flag queue / meta / chat / etc. (again, that's not a hard rule or anything, just a guideline off the top of my head).
@ABcDexter I'm 15 right now and was 13 when I was appointed on PPCG, and I know we've had several other moderators of similar ages elsewhere on the network. So yes, I would say it is :)