Children who start later rather than early are usually successful, and have very little retention, therefore avoiding an unfortunate stigma weave never seemed able to eliminate. After third grade, many of the children who are developmentally young also develop social problems. So we're not only looking at academics here, but the quality of relationships as well. Most elementary teachers do try to individualize, and to gear their program to all the children, the younger ones as well as the older. And so, often against their best judgment they will begin to push those young children.
Standardized tests are another factor that cause teachers to push.
With all that said, I have to say that there are always exceptions to any rule. Those children may be ready. If you have questions, an experienced preschool teacher can usually give you good advice. Some schools offer developmental assessments on request. A child can always be moved ahead at a later date.