Older people grew up writing less than younger people have, because of texting, so they’re more accustomed to taking their time with the proprieties of grammar. Younger people began using grammar as a tone marker differently from how it had previously been used, so they tend to see a bigger difference between “no” and “No.” as an answer to a question than older people do. For younger people, the latter tends to seem more abrupt and final, which could come across rudely.