welcome to the clubYep.
I would bet that's related to the development of the "sense of self". Young kids don't have as strong of a concept of identity, so it's easier for them to connect with others.
I think, beyond that, that there may be an underlying, motivating sense of the 'impending judgement' of one's actions by society. Whereas, a child in a lucky case may be essentially carefree, an adult is always subconsciously on the lookout for "person x in situation y may judge me z if i do that". Autistic people are often free of that: social comprehension, hardly unique to us as a species, is at once a freedom and a curse.
Growing up, a very close friend of one of my parents had a son who was autistic and slightly younger than me. They lived in the country, and we would visit them approximately annually. When it became apparent that he had autism, all of his behavior became explained through the lens of autism. He would run through fields, climb trees, chase animals, play with model tractors, make loud noises, inquisitively inspect minutiae of nature. One of his parents was an unrelated medical practitioner, and the pair of them dedicated immense time and energy to attending all possible events and reading all possible books to understand the nature of their son's condition. Because they could afford it, the father wound up spending all his time every day with the son, attempting to use various techniques to influence his behavior. Through the stop-motion perspective of our occasional visitations, it was apparent that slowly, he was achieving success. The son began to talk, and was exceptionally skilled with language, song, performance, drama and all of those qualities that are inherently social... precisely the elements that were perceived as missing from his life before-hand, but present in everyone else's "normal" experience of the world.
I made up my mind then and there that autism was not a bad thing, it was possibly a good thing... a rare situation in which children may be shielded from the tremendously judgmental, often vindictive and horrible, backward and limited perspective of society at large and its collected habits and wisdom, and instead allowed to develop freely through raw curiosity and thought. It is as if they are locked within their own Eden... and at least some of those who do eventually escape are far wealthier for it.