Some people do younger but I would say the most usual earliest age is 15 months. Don't know what other people would think about that. If they are small or the bull is big, then leave them longer, say 18-22 months even. I'm talking non-chondro.
We play safe and usually leave them until 18 months whatever their size, but that's just our choice.
You will hear various opinions on this. Consensus seems to me that the earliest is about 13 months, most people prefer 15 months, and some wait till 18 months. I personally go with the 15 month mark, that way the mom is 2 years old when the calf is born. If you have an exceptionally mature or immature heifer, you should adjust accordingly.
Growing up with beef animals and working on Dairies, I have always heard to breed at 15 mths so she calves at 2 yrs old. Earlier breeding is practiced by some but it tends to stunt the mature growth of the animal so they never grow to their full potential. I have heard some may intentionally use this to decieve people of what the true height potential of their animals are, especially with miniature breeds.
Kit Pharo gives a very good presentation about the benefits of calving when the grass is right.
That said, let's suppose that you manage to have a heifer born at exactly the right time. You'll want to breed her at 15 months so she will calve at the same time, exactly two years after she was born.
So, does this mean by the answers above that everyone has the facility to separate their bull from the rest of the herd? I don't. What then? What is the normal age for a heifer to start heats? I have my first experience with this with a Dexter/cross dutch belted born this past June.