toad,
Breeding at 14-15 months gets these girls on track to hopefully get on with the task of paying their way.
For most British/Continental breeds, sutures between pelvic bones fuse at about 27 months - so if you can get 'em bred and calve 'em out once before that happens, there's a little more 'wiggle room' for 'em to push a calf out through. 18 months is about as late as I'd care to wait.
I know plenty of the old-timers 'bred at 2 to calve at 3', but just because that's the way grandpa did it, doesn't mean it's the right way.
I know some folks advocate breeding to Corriente, Longhorn, Jersey the first time out, to help ensure that the heifer calves easily, loves that calf and learns how to be a good mama - and they count that calf as a 'coupon' - and I'm not saying that there's anything wrong with that; but if it's gonna be almost a throwaway calf, I'm gonna use a high CED/low BW black Angus bull, and hopefully end up with something that might grow off a little better - or even be worth keeping as a replacement heifer. I'm just saying...
Now, I REALLY like Jovid's Red Polls - and don't know much, if anything about calving ease in that breed (my dad used to raise 'em long before I was born) but you might get some red calves, if your cattle have red in their ancestry. I prefer red cattle, but my market prefers black, so thats the main direction I've taken my herd.