For me, I was walking/talking/potty trained really early... But apparently the staff at my nursery school (I would have been two or three) expressed concern about the fact that I refused to socialize with other children. Seems I preferred to hide away somewhere and stare at picture books rather than play with other kids. So, maybe there's that? I remember being taken out of class in first or second grade and taking all these tests by myself in the school library - my mother told me it was because they thought I might not understand English well because we were immigrants and I was so quiet, but I wonder now if there wasn't more to it - especially since I remember my teachers praising me for being so smart at that same age. It's hard to get facts about the past out of my mother, what with the language barrier (she does not, in fact, understand English as well as she likes to think) and her tendency to change a story with each telling.
So I don't know about ALL, but I suspect most of us do have development delays, but for some of us it would be social, specifically. A subtle thing, easy to dismiss as extreme shyness, as it was with me.