It's unfortunate that innocent parents of older-appearing children are often suspected of lying. The suspicion, however, rather than being a "judging a book by its cover" reaction, I think is more often a case of reacting from experience -- often from long experience. I can testify from experience that lying parents are very common at ticketed events that offer children's prices, and even more common where junior's tickets are available.
The sad reality is that movie theaters, theme parks and honest parents of large-for-their-age children are all victims of the lowlife parents who deliberately lie about the age of their children. It's a bad situation all around, and one which has no cure other than for businesses to require proof of age. Which, as we have seen, is unacceptable to most parents, whose usual reaction to skepticism about their child's age is anger.