Like everyone, I noticed Sir Burnevere is the oldest of the 5 dragon knights. I personally headcanon him 51, while I headcanon the king 60.
Headcanon Sir Burnevere came to Camelhot at the age of 18 and became Sir Allfire's, who was 27, squire. Then, when Allfire became king 2 years later, he became Leodragon's squire, ang got dubbed quite quickly, like one year later. Here, King Allfire is 30, and Burnevere 21. I like to headcanon they're good friends, got very along when one was knight and one was his squire, and they would never every betray each other.
The problem for the king is he aged kind of badly, while Burnevere remained tall, thin and muscular. Poor king. Nature is a bitch.
Lol, I tried to draw Sir Burnevere with the season 2 design yet wearing the clothes he wears in season 1. Looks odder than the other way round.

Blazing dragons, King Allfire and Sir Burnevere © Terry Jones
Drawing, idea, theories and Burnevere's younger design © me