Instant Friendship

Was watching the shorter of the short people in a context the other day where he came together with a couple of other kids he had never met. The instantaneous reaction was, "Oh my god, you're a kid? I'm a kid, too! That must mean we're friends." And so they all were.

When do we lose this? Is it adolescence? Earlier? Later? Is it good that we lose it? Is it naive innocence or artificial barriers we put up towards fellow adults?