Oldest kids have it rough

First born kind of implies there's siblings

Yeah but I'm just sayin what if there were no siblings

It's a well-known fact that they become selfish. For instance they take a post that's not about them and.... (don't worry I'm kidding :) )

Only children are the meanest. They never had siblings to help them develop their clapback immune systems so when teased they don’t rib playfully, they tear jugulars ferociously.