We took DS for the first time when he was 22 months and it was great. He never had any fear of the characters and loved seeing all of them. Plus, at that time he was facinated by lights, so while we watched Specto he was sitting on my lap and kept turning around and giving me kisses and saying I love you. For me that has been the most magical moment at WDW.
So I don't think any age is to young. Each year DS has had a great time and I have been surprised at what he has remembered about the previous trips. He will bring things up on his own. Today he said he wanted to see Mickey fight the Dragon again. So Fantasmic is a must do for us in October.