Umm hello? in the final episode we *SEE* Slayers aged from 5-45 or so being called. We're shown it, for a reason, its even recreated at the beginning of one of the comics in a flashback.
The Slayers as they are now, are of all ages. Not all of them can or could fight, one of the Slayers called in Chosen seemed crippled and now was walking, badly but still. Another was severely overweight. Some where white, some where black, tall and short. Basically being female and being Slayers was all some of them had in common.
And since they had a hard enough time finding a few dozen Potentials in Season 7 and at the beginning of Season 8 the main army with Buffy only consists of 500 of those 1800, with much smaller groups in other countries, even a year and a half on they haven't conscripted half of them into active duty and some don't even want to be good.
Factor in loosing Satsu and Renee from the roster and they're down two main Slayers already, one dead one resigned.