Presumably the Watchers have access to some manner of divination to determine whom may be chosen. Chosen means either God or Fate, neither of whom play dice with the universe, so this status ought to be knowable at a very young age, but for poetry's sake, I agree that puberty is as good as any. It works for the X-men. As for an age limit? I would guess the Slayer is only described as a Maiden, so, no older than the end of adolescence. Beyond that, the series had absolutely no world-building. The other issue, if you went too young on the actresses, you would either have to hire tutors or pay for emancipation decrees, which is a no-no. 18-24 year olds can easily play teenagers. Of course, the Potential pre-activation process may have been halted as part of the disturbance in the force that Buffy's revival created, so no more young girls?