I have heard about the "fusing" of the pelvic bones. However, we have much better success with our heifers calving them in the 27-30 month range. They have the maturity to settle down and calve and keep on going about their business. Few if any in the past 20 years have given us any problem with accepting their calf, and the few that I have had to graft a calf on have seemed pretty well to take it in stride. Maybe 2 or 3. They just want a baby.
They have more body condition. Because we do not feed our heifers to grow very fast, they take a bit more time to get to where I want them in size. We don't push them, and don't get much grain except when they are first weaned they do get supplemented. By the time they are 12-14 months, they eat hay and grass, with once or twice a week grain to call them into the pen. Most are in the 900 + lb range when they calve. Most of our cows are in the 1050-1200 when mature. They don't seem to lose much body condition when they are milking and tend to rebreed easily. I use an easy calving bull on them and haven't pulled a calf in years from a heifer. They just spit them out and they get up and nurse and go on.
We use plus weight bulls on our cows and don't have problems with calving either. Calves will run from 75 to 90 lbs on the cows; 55-75 lbs. on the heifers.
Everyone has to do what they feel works for their system. We do not run our heifers with our cows until they have their 2nd calf on the ground and are incorporated into the mature herd. I don't want them having to compete with some boss cows when they are trying to eat, grow, feed a baby, and get bred back. We are lucky enough to have many rented pastures, some smaller sized like 10-30 acres so it is easy to group them to take advantage of the pasture and keep them in a group that they will succeed in. Once they have their 2nd calf on the ground, they are "grownups" and can hold their own in most any group we have.
Occasionally we have held a heifer back to get her in synch with the rest. Often they are bigger, older, do get bred to the easy calving bulls, and if they have the size and the calf is doing good, may go with the mature cows with their first calf. There have been a few that have gotten bred sooner, and they never seem to get their full size. Usually do a good job with their calf, but it seems to take more out of them. But again, we do not "feed" them except what they get at pastures in the grazing months. We do feed some silage in the winter to weaned calves and the fall first calf heifers if they are at that barn lot, and the grandma cows get it to help them keep up their body condition and get the calves through the winter.