These things are in the nature of "tips" or "preferences". For our family, it makes no sense to stay on-site. To us, vacation accommodations are nearly as important as the destination -- we value our "down-time" a lot more than others who are on vacation. Disney's "home away from home" options aren't the best in the Orlando area, in our opinion. We prefer some of the Marriott properties, especially the Grande Vista. So why pay a premium to stay on-site in a unit that isn't as nice as one 10 minutes down the road?
If you're the type who values the "magic" of staying on-site and you want to be as close to the Mouse as possible, staying off-site to save money may be an unacceptable compromise.
As for how old your children need to be to appreciate Disney, all I can say is we took our oldest daughter when she was 15 months, and she did great. Does she remember that trip in particular? Heck, I don't know. All I know is while we were there she was happy, and handled the parks very well. It was a great
family vacation, and I'm very glad we chose to take it.
On the other hand, I've seen situations with families in the parks where I'm fairly certain that perhaps the parents regretted not waiting another year or two before bringing their children.
No "expert's" tips in a travel publication are going to apply to every situation and every family. But it is interesting, and sometimes informative, to learn how other people think about planning
Disney vacations. I'm guessing that's the reason most people are on this board in the first place.