We took my 5 month old niece on PotC, Small World, Peter Pan, Winnie the Pooh, Jungle Cruise, Dumbo and Alladin during her first trip to WDW last year.
She didn't react much but we were gratified to see her smiling during some of the rides and looking from side to side in others.
On a side note, she is nearly two now and our last trip (last week) was the first time she has ever noticed the attractions. For example, we could tell that she saw the 3D (when she would leave her glasses on!) and she was scared in parts of the HM.