Around 3-4 I found it so strange that he would still basically try to parallel play at other kids houses then I realized he had given up on the social side and preferred to focus on their stuff.
This was our experience when DD started preschool. She wasn't interested in the other kids (never mentioned a single other child) & the teachers asked us after a term if she had a speech impediment because she never spoke. But she loved to go, they had an enormous array of interesting materials to explore. It was quite a process to get her to engage in group time, follow instructions or do anything that interrupted her exploration of what she found interesting.
