I suspect the entire policy of making a teenage girl a slayer is partly based on financial exploitation of a minor.

YES - as well as the fact that women and children were literally the legal property of men, whoever was their "guardian" at a given time. A boy might declare his independence and strike out on his own at age 18 (or whatever the age was in any give period); a female "adult" was always a considered a minor.

I'm surprised that neither canon or fandom explore that very much, it seems like such a rich topic for consideration.