Usually kids engage in solitary or parallel (playing next to someone, but not interacting) play during the toddler years, and then start to engage with other children around the age of 3.5. of course, for some children this happens much later. Children should be encouraged to play with others their age early on, so that they can develop social skills. Sometimes kids are shy, or aren't sure how to begin playing woth others, so they might need a little help to get an "in." If your child is past the toddler years, and entering (or already in) elementary school, you may want to look into why he or she is not engaging. Some causes of this kind of behavior are Autism and Asperger Syndrome. If you are concerned, you may want to get your child evaluated so that you may help him or her out of her shell. I recommend lits of play-dates, preschool, summer camps, and a lot of communication with your child's teachers and other caregivers.