Preschool is not mandatory. It can be a valuable stepping stool if your child is going to be going into the school system, as it can help them prepare, however you do not need to worry about a truant officer of police officer being sent out. The age at which children are required to be in school varies from state to state, but most are between 6 and 7 years old that they are required to start. Of course even then, most states have rules that will allow you to homeschool if you meet their requirements (again these vary from state to state). If you'd like to tell me your state, I can see if I can locate your state's minimum schooling age for you.
The state of Colorado just changed their minimum compulsory school age this year. It was 7 now it is 6. What this means is that until your child is 6 s/he is not required to attend school of any sort, and thus you do not have to worry that you are violating any laws whatsoever. So, as long as your child starts kindergarten before s/he turns 7, you are fine. In general, I personally would say go with your gut, if you do not feel your child is ready, it's not necessary to push them into it. Try to get him/her into some activities where they are following the instructions of other adults and having positive interactions with other kids... things like gymnastics, swimming, etc are a great way to start breaking the ice.
You can read about the law here:
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