I don't think the Council would be ruthless enough to just kill them once they pass 20. :-( What a horrible thought. Though I wouldn't be surprised if at particularly brutal periods of history, they might have. I can see a Potential/former Potential, say, being burned at the stake as a which because of her abilities.

I can't really see the Council allowing them to just go out free into the wider world; they had too much information, not to mention a skillset that wouldn't really suit them to be a housewife or farmworker, and would raise questions about how exactly they learned 53 different ways to kill people with their bare hands before they were 16. Very good point.

I always assumed that they didn't really understand the process by which someone becomes a Potential, but they have 8,000 years of observational data which helps them to identify possible Potentials. I guess I assumed that because they don't seem to have any problem finding the Slayer once she's chosen, they should be able to find the Potentials pretty easily, too. But perhaps there's a spell to find the Slayer specifically because of how much more powerful a Slayer is than a Potential?

I love the idea of the magical equivalent of a recessive gene. Absolutely.