Even if demons are long lived, if they mature quickly and procreate at the same ages as humans, there can still be quite a number of generations. Especially in a medieval society where people tend to have children younger than today.
Say about, 10~12 generations in 300 years. That's quite a lot. And even if they are long-lived, they could have a concept where the monarch abdicates to a heir. Demons respect the strongest after all, so they might not wait to pass on the crown until they're elderly.
That being said, that's just speculation. I've not seen any evidence demons live longer than other races in this story, and I haven't read further than this in the novels.