We can attest by experience that heifers can not only come in heat at 4-6 months they can be bred as well. Our first "teenage pregnancy" we noticed because my husband was driving home from work just before dark as she was "walking funny" across the pasture. So in the pouring rain we watched her work and work, finally we herded her into the barn and as she stood outside the barn, surrounded by part of the herd (helped keep her calm) she pushed, I pulled, my husband helped hold her up and the calf piled out on top of me. We stuffed them into the barn and she instantly started doing the mom-thing, talking, licking etc. It wasn't a hard pull, just awkward because the calf was darn near on the ground and not half-way out!
We have had a couple more since, the youngest one was bred at barely 5 months, still on her mom - so far no losses of either calf or cow. The first one was the only one that we assisted, none required any other help and haven't missed a year since. That year they are a little smaller frame-wise and skinnier than their buddies but I haven't seen any problems otherwise. We have been pretty lucky, there are so many things that can go wrong in regular calving, much less when the cow is so much younger the loss of or injury to a good heifer doesn't balance out with another calf. We started pulling our bulls much earlier and weaning a little earlier as well - we may even give the oldest girls an abort shot or preg check them when we Bangs them.
