There doesn't seem to age limitations for J class passengers. My return flight (AC898) in J a few weeks ago had two crying, whining, screaming kids up front for 11 hours of our flight. It got worse when the parents decided to change the diapers right there in the J cabin, which made the cabin smell awful.

I usually don't have too many problems with well behaved kids in J, but when it gets bad enough to annoy the rest of the passengers, the flight can be unpleasant.