Maybe, Mavis died young.
That's not what I'm questioning, I'm questioning how she was the Guild Master when she was so young. I just expect Guild Masters to be older and more experienced with magic. That's why I'm thinking maybe she's much older than she looks.
By the way, where did you get the name "Fairy Camp" for the fourth OVA? Both Wikipedia and the Fairy Tail Wikia say there's no known title for it.