At 3,4, and 5, my DS thought everything was real. At 4, he even thought we actually took a flight to space on Star Tours. That being said, he had a Batman birthday at age 4, and a little girl in his class already told him it wasn't real (so some kids believe longer than others).
My DS is 8 now, he still totally believes in Santa, but knows Disney characters are not real.
He is at an age, though, where he can read trip planning books and he is so excited to trip plan.
ETA: I even neglected to get him an autograph book (got his little siblings one) and he is not too old for that because I got an earful LOL. To be fair, I did get him some pins for pin trading. He is stoked about the Sorcerers of the MK and Phineas and Ferb in Epcot.
He also chose an AkA cars suite for our room
Oh, and he REALLY wants a
Disney cruise...not in the cards for us right now, but just letting you know how great the excitement for that is