Not definitive, true, but it could be an informed guess based on Kennedy having read the records of previous Slayers and taking note of their ages when they were each Called. She's one of the Potentials who were taught from early childhood by a Watcher, like Kendra, after all.
(Frankly, I think even the Council operated more on guesswork than science when it came to finding Slayers)
True, but I'd think the fact that even Kennedy sounded unsure points to it being a more general belief than a hard and fast rule. Most likely, yeah, most Slayers are called younger. However, we don't have anything concrete to say that there hasn't been an occasional older Slayer called.
no subject
no subject
no subject
no subject
(Frankly, I think even the Council operated more on guesswork than science when it came to finding Slayers)
no subject