I think it depends on the kid. My son is the most easygoing, happy baby ever on the planet. We are taking him next year at 18 months. I'm planning to do lunch at Garden Grille. I hadn't planned to take him until his 3rd birthday, but his face lights up whenever he sees Mickey on anything...TV, a stuffed toy, anywhere. Yesterday, we were birthday shopping for my nephew and he tried to pull a Mickey puzzle off the shelf at the store. He adores Mickey so much, I'm pretty much taking him expressly for the experience of getting to meet the "real" Mickey. I truly think he will completely enjoy it. I would take him tomorrow if I could. Of course, if his personality seems different by the time our trip comes, I'll reconsider.
I'll also mention that my sister took her son this past February when he was 18 months and they did a meal at Crystal Palace. The pictures of him hugging Eeyore are priceless. He loved it.
You know your child best. If you think your kid will like it, I say go for it! The other thing I'd say is not to be upset if it doesn't work out. That's how I view outings with my son. If he likes it, great, if not, oh well. A good attitude can go a long way.