Going to preschool is a big milestone for a toddler. Infancy is quickly passing and he or she is steadily growing to become a person who is more independent of his or her parents. Starting school is the first step to going to the "big school".
Many schools accept toddlers around age 2 1/2, but as Baby Center points out, this doesn't mean a child has to start school at this age.  Many children decide not to start their kids at preschool until age 4, or the year before they are scheduled to enter kindergarten (depending on what birthday schools use as a “cutoff” date that allows a child to enter school. For instance, in the United States, this varies from state to state or even within states).
Deciding if and when to send a child to preschool is something many parents agonize about. There is no set "right" time, it's important to consider your family and your child's needs first.
Rather than looking to age to decide when your toddler should start school, it is perhaps of more value to determine if he or she is physically, socially and emotionally ready to go to school. Before making the decision of whether or not to send your little one off to preschool, ask yourself a few questions.
What is Your Child’s Level of Independence?
If your child is independent, this is a strong sign of readiness. Since your child will be in with a group of several kids, he or she will typically need to be able to wash hands, self-feed and be able to generally listen and follow directions. Toilet training is another consideration. While some schools will take children not potty trained, there are schools that do require it to be completed before they'll accept a student.
Self-Regulation and Socialization Skills
In preschool there are expectations of children being able to follow directions, stay on task, and an ability to have an age appropriate attention span. Also, there is a high level of socialization with other children. If your toddler doesn't appear ready to be able to handle these types of directives or still has difficulty or is overwhelmed in group settings, it may be too soon for him or her to begin school. Consider though, that school can also help with development of these issues too.
Many kids easily adapt to group situations, but other kids may need a little more time or help transitioning.
How is Your Child’s Stamina?
Can your toddler handle being in school for a few hours at a time? Stamina is an important consideration. Some preschools limit classes to two to three days a week for only a couple of hours per session. Other schools may have daily or full day sessions. If your child seems ready, but not for a daily school, consider looking into a preschool that has limited classes during the week.
Does Your Toddler Have Any Separation Anxiety?
Separation anxiety is an issue many parents face at the toddler (and even older!) stages. Before you make a decision about preschool, ask yourself how well your child handles being away from you. If your child has a lot of separation anxiety, you might want to start your toddler off slow, perhaps joining play groups, gymnastics, dance, art classes, or any other age appropriate activity).  Another approach is to allow him or her to spend time with friends and family while you are not there before plunging into school. In some cases though, it's the parents who have the higher level of separation anxiety and, in those cases, it's often said parents should remember it is usually harder on them than on the child. 
There is No One Size, Fits All Answer
Sending your child off to school for the first time is sometimes a difficult decision. There is no one size, fits all answer because all kids and families are different. And some families decide not to do preschool at all. If he or she is not ready, you can always pull out and give your child’s spot to someone who may be on the waiting list. The best way to approach it is to evaluate your toddler and go with what feels right. It is OK for him or her to not start preschool immediately if his or her readiness is not there, however, if it is, preschool can be an enriching experience.
Children build on skills already learned at home and also learn some new ones at preschool, helping to provide a solid foundation for the eventual transition to kindergarten.
Starting the Process
You've asked yourself all the questions, considered what's best for your family and have now made the decision it is time for your little one to head off to preschool? The next step is picking the school that is right for your child.
Unfortunately, due to competition for enrollment in some areas, many preschools these days begin to take applications and set up waiting lists many months, or even years, before the school year even starts (and an awful lot can happen in the course of those six to 10 months or longer!) That being the case, you may want to decide to apply to the preschools you want your toddler to go to even if you are not sure if your child is ready to attend. It’s a good idea to play it safe and get on the waiting lists of the school you are interested in, this way your child can attend if ready to do so.
Going to preschool is a huge step, but doing your homework and assessing your child will help you decide what's best for both your little one and your family.