My kids are 9,12, and 14 and all play Warmachine/Hordes. My 12 year old son plays Malifaux and takes his share of victories against adults. His first crew (his idea) was Collette (not the best master to start with) and he had no problems, his favorite is collodi. I might recommend one of the more straight forward crews for any new player (Lady J, Seamus-if you are ok with the afore mentioned zombie hookers, Victorias, Ophelia, Perdita...).
Good luck and have fun