Pre-School, ages 2.5 to 4 years, is when you start to see a more regimented schedule appear, and when the curriculum starts to shift from the focus being on the child to the world around him or her.
For example, hopefully, by this age, potty training and hand washing foundation and procedures have already been learned and laid, so we don't have to spend time on that, we can spend that time flexing our attention muscles instead and learning to sit and pay attention in Circle time for longer periods.