Why do people see me as far older than 19 when I type the way I do sometimes?/Why do people think full stops are rude?

This is a 2-in-1 question, I suppose. I type the way I do. I’ve always typed this way, but I’ve noticed when interacting with people (not on here) that people always think I’m far older than 19. They think I’m in my forties or fifties.

Also, I tend to type using full stops, which people may think are rude. When I’m typing a full sentence, though, I end it with a period. If I say, “He’s being an asshole,” (with a period), I mean that as a fact, not out of anger. It just happens to be ended with a period since it’s a sentence.