I know I'm responding to this way late, but I have a different (and, in Torben's word, thorough) answer for you. ;-)
Disney in particular actually has two "lands" that cater to younger ages and such a variety of rides that it really is fitting for almost any age. Between 2-4 fantasyland and (3-7) toon town are specifically geared for younger ages and if a family was to stay at a near-by hotel and focus mainly on these areas a child could easily enjoy the park and still get an afternoon nap. I don't think that even at this age, he will run out of rides to go on - in fact, it is the teens that might feel most limited because so many of the rides are really too small for them. Plus for someone who is 2-4 there is just a lot to see and take in even not considering rides.
There is stroller parking throughout, the restrooms are set up with changing stations, and most of the restaurants are pretty open for all ages. At the same time, if a crowd and costumed characters will be overwhelming, over stimulating or just scary, I'd wait. Alice had a great time at two and again at four when my parents brought her. There are also other age-appropriate rides scattered throughout the park, like finding Nemo near Tomorrowland and Winnie the Pooh just past Splash Mountain.
Between say 4 and 9, the magic of the fairy tales and their belief in those tales is really fun and the number of raids available expands somewhat (they also are less needy of that afternoon-nap). This is the time when posing with characters, collecting autographs and doing things like exploring the tree-house (now Tarzan, used to be Swiss Family Robinson) and castle interior are super fun. At this age, they can handle rides like the pirates of the Carribean and might even enjoy taking in a show.
As kids move into their pre-teen and teen years, the "magic" might be gone, but hundreds if not thousands of high-school graduates flock to the parks every year for "grad nights" and still have a blast on the faster, more thrilling rides like space mountain and thunder mountain railroad. By about 8 or 9 most kids should be tall and heavy enough to enjoy all the rides if they wish (I think).
So, I'd say just about any age is fine (though pre-four he won't remember much of the trip in just a few years because of how their brains grow). Some tips though - go in the off-season, lines are shorter. Use the fast passes system so line waits are shorter. Start by heading to the furthest area of the park from the front gate you want to visit and work your way forward (again, to avoid super long lines and wait times). Check into the lunches with the characters - you reserve a spot and go to eat a specific time and one of the Disney characters will come and sit with you for a short time while you eat - extra cost and must reserve in advance, but if you don't get to go often - REALLY fun for your little one. For updates about changes at the parks check out Fodor's. I'd also recommend bringing grandma and grandpa along (if available). They'll love seeing your little one take in the sites too and you can take turns with him so you can enjoy a couple of the "big kid" rides too.
I know a lot less about California Adventure and Epcot Center although I know most of the "Pixar" stuff is featured at California adventure so if you are headed to the Anaheim site, you might prefer a park-hopper pass and a couple of days so your little fan can enjoy seeing all of his favorite characters.