I totally agree - you are on your vacation! If they want to at 18, I would be thrilled!
My kids never did autographs (I'm glad, as I'm too impatient to wait in lines for them), but I think if you are at Disneyworld, you should be like Peter Pan and not grow up! Where else can you see adults wearing silly hats with ears, eat huge turkey legs, and ride Dumbo?