We got back 2 weeks ago and our youngest was 15 months. He loved it! We wound up going at the pace of our 6 year old and the younger two dealt with it fine. The little one's favorite ride was Buzz Lightyear and we rode it many times. There was so much for him to look at, he really was happy there.
We even took him on the the family raft ride at Blizzard Beach. There was no height restriction on the ride itself and the chair lift said 31 inches. He was 33 1/2 so up we went. The people on the line were pointing at us...they were either saying "look at the cute baby" or "look at that crazy mother" LOL!!
He did adjust better to the unexpected as the week wore on. For example, he cried during Mickey's Philharmagic which we did on the 1st day. The water sprayed and the music was loud. But by the end of the trip, he was used to surprises, the characters and the stimulation and he loved it.
Enjoy!