My nieces were 1 and 3 when they first rode POC. The 1 year old loved it and still considers it her favorite ride at 3. My other niece is now 5 and she HATES it, doesn't even like to be near the entrance because she's afraid we'll make her go on.
I gotta tell ya, in September my sister and I took turns taking my niece on so one of us could stay with the older one. When I went on with her there was nobody online, we walked right on and had a boat all to ourselves with nobody on the boats before or after us. It was kind of freaky! When we got back off I told my sister I sort of understood why Bri is so scared of that ride. She said the same thing happened when she rode before me and she had thought the same thing. I love POC, but being in there totally alone was a bit frightening, so I could really see why young kids might not like it.