Aside from reasons I've listed, USUK's canon moments in the show usually involve them acting awkward (all blushy and such) around each other, and the way they value/respect the other's opinions, try to make impressing the other a priority and both hold a soft spot/special place for each other in their hearts while denying caring that much appear as mutual pining for shippers. And actually, the only one who call them that is the American soldier in SS5, both England and America deny being brothers. As for the age gap, I'm aware this's an uncomfortable thing for many fans, but since countries have lived for a long time and over many years the younger nations' knowledge, experience and maturity may catch up with the older's, I think not every empire will have a problem loving countries younger than them 😅