Michael Schoch, Noodle Intern
The ages and deadlines for enrollment vary from school to school. Your best bet is to find your local school district's website, or to simply call the school directly to get up to date information.
There are daycare programs for children ages 0-3 and pre-k programs for children 3-4. At many schools, incoming pre-k students must turn 4 years-old by September 1st of the year they will attend. This is only a general guideline however and you should always double check with the school at which you're enrolling.
For more information about pre-k, I recommend visiting some of Noodle's topic pages on the subject. You can find expert-written articles covering everything you need to know about finding, preparing, and paying for preschool.
Another great article that covers the nuts and bolts of school enrollment is Enrolling Your Child at the Right Place and Time.
You can search for preschools on Noodle's listing; most profiles contain contact information which you can use to learn more about deadlines and requirements for enrollment.
Best of luck in your search!