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.
For the first question, you might find answers more helpful if you ask the aforementioned people that you’re actually interacting with.
And regarding the second question, it’s probably related to the first. Young people who grew up texting and using social media have adopted abbreviated writing styles. Older people using the same technologies tend to retain more proper forms. Of course, people aren’t monoliths. There’s plenty of young people writing in complete sentences and plenty of older people utilizing short form words.