I'm pretty sure they have the 4 yo cut-off for liability issues, and cannot bend the rules........A couple years ago, when we were dropping our son off for this, they actually had a little girl get off the boat (she didn't want to go anyway - she was scared to death!) - because they had asked the little girl her age - and she said "3" (Because she was) - the parents, when confronted, said that they didn't realize that the age limit was 4 (We were all standing on the dock, waiting to wave goodybye to our "pirates")
(Note that they ask you that age question when you sign the children in)............
Another thought-
- My Son, who had just turned 4 about a week prior (we were at WDW celebrating his Birthday) - had a miserable time on the Pirate cruise - because most of the other children were older - He sort of got crowded out (pushed to the back) of most of the treasure hunt activities ---- The CMs try to make sure everyone has a good time - but there are only a few of them, and maybe 20 children? My Son isn't shy, didn't mind going off without us, and had always been involved with older Children......I just think it might have been a little overwhelming......We are going to try it again this year after he turns 6.