there's also no proof that a Slayer DIDN'T live until a ripe old age of...what? thirty?
as far as we know, Robin's mom was probably the oldest Slayer on record...she was definitely older than 18 when Spike killed her, considering Robin was at least 5 or 6 by that stage. Buffy has lasted pretty long too, 8 years at the time of 'Chosen'.
and if younger girls were Chosen over older ones, then what about Kendra and Faith? they were definitely not little teenage girls when they were Called, in fact, i think most people believe they were both older than Buffy at the time of their Activation. if Kennedy was already in training at that point, why wasn't she Called instead? Kennedy was, what? 19 or so when she showed up in Sunnydale? and she's known how to use a crossbow since she was 8 years old, meaning she was in training for at least 11 years, which would have made her around 13/14 at the time of Kendra's Calling, and 14/15 when Faith became the Slayer. why was she over looked, twice, in favour of older girls if age is taken into consideration?