If a twelve year old boy and adult woman get into a fist fight, who is the aggressor?

[tailfins]

Let's us suppose it's two siblings. The adult women slaps the 12 yr old boy for convincing their mother to buy an expensive gift instead of giving the adult woman rent money. They boy punches her in retaliation and a fight breaks out. It's in where the boy lives and the adult woman is a visitor.