I've started my 3 kids in Quetico when they were 11, and we did not do trivial routes.  Looking back it could have been younger & they would have been fine -- but it would have involved more work for me.  

I think that's what it comes down to -- for me it wasn't a guys fishing trip anymore - I fished, but the focus changed & you spend a lot of time helping your kids.  That said, the first trip with a kid was my best trip ever & I would not do it any other way!

So what's that mean for you?  Whatever age you decide is 'old enough' start with a trip that's not too aggressive & be very flexible!

-Tom