First time we took DS last year, he was 2.5. He loved having the characters "write their name."
We have a Disney autograph book which actually has lots of pages. We only live an hour from WDW, so have been back since that first time. He is now 3.5 and loves seeing the characters, and will actually tell me when he needs his autograph book and when "I already have that one." He still visits the character, just doesn't ask for the autograph. We have the characters always sign the right page, and take a picture of DS and the character and glue it into the book on the left page (an idea I got here on the DIS). This makes the autograph book even more special.
Last time we went, Jessie and Woody from Toy Story were together. He, for whatever reason his 3-year old mind had, only would let Woody sign his book. Now he cannot wait to go back because he regrets not getting Jessie's signature!