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.
To me, sentences ending in a period feel immutable, and without nuance, but sentences without a period feel incomplete, or up to change. Without periods it is almost a way to say, “this is what I think right now, but I might reconsider.” So, it’s not that periods are rude per-se, but it may appear that you’ve made up your mind and are closed off to interpretation. Sometimes I intentionally remove periods or turn it into an ellipsis for exactly that reason. It’s just way too easy to misinterpret people’s intentions through text for me not to type in a way I think reduces misinterpretation.
As for being associated with older people… anecdotally speaking, my co workers sound like they were taught that there is an immutable, proper way in the world, and so they express themself in that proper way. Nothing wrong with that really! Once I get a feel for their personality, I find it kind of endearing :)