I really don't know how old Bonnie Story is. It's possible that the age isn't clearly mentioned or it could be found in specific documentation or related materials. You could try looking there.
Freddy in fanfiction sometimes has a sense of humor. He can crack jokes to lighten the mood, especially when they are in a tense situation. Bonnie is often portrayed as intelligent. He might be the one who comes up with clever plans. In a story where they need to outwit an enemy, Bonnie's intelligence shines through. Also, their loyalty to each other is a common characteristic. No matter what happens, they always have each other's backs.
Friendship is a common theme. They often start as strangers or even foes and then develop a friendship. Another theme is adventure, as they explore the pizzeria together.
Yes, there are some. You can try searching on fanfiction websites. Some might focus on their unique relationship in the FNAF universe, like how they interact in the spooky animatronic world.