In the scene where Josh (Tom Hanks) is having dinner with his girlfriend, he tells her he actually a child and also tells her twice that he's 13. When they are at Seapoint on the pier, she asks him "So how old are you? 15, 16?" He already told you he was 13!!!
Character stupidity or inattention isn't, IMO, a slip up. It could very well be that the details weren't really registering with her while her mind tried to wrap itself around the idea that this guy was really a kid.