"Ideal" depends on your kids.
My girls were 4 (we embarked on her birthday, actually!) and 7, which many might say is a great age. But for us it didn't work out as well as I'd hoped. First, we couldn't get switched to early dining so we missed all of the dining rooms because they simply couldn't wait that long or stay up that late. They don't nap and wake up early no matter what you do so it's bed by 9:30/10:00 and even that's risking a major meltdown. My younger DD, in particular, likes to talk/sing herself to sleep so the two of them in the same room ended up being interesting. And DD #2 absolutely HATED the kids club and refused to go after the first day. So no Palo (we had to cancel last minute), no adult beach on CC (we managed to get her to Scuttles Cove only to be paged to pick her up after less than an hour), no Cove Cafe/adult pool, no shows (the girls weren't interested in seeing them). Not that it was a bad thing to have more family time, but it definitely changed our overall plans for the trip. We spent a day at MK after disembarking, and they were FAR happier and much more interested in that, so while I'm taking them back to Orlando in April we won't be planning another cruise for several years!