I started taking my kids when they were 9.  I agree with MK, the younger they are the more work it is for you.  More portaging, more fising fishing problems, more cooking, etc.  It is a different trip than what you are probably used to.  But it is all worth it, I figure when I am old and grey, he is going to have to pick up the slack for me.  My oldest is now 16 and he can hold his own and carry "big" packs or the canoe.  The next couple trips I plan on getting him more involved in navigating, camp and fishing spots location, camp setup, etc.