For me, it's not just the feeling that it was long ago. It has the same kind of unique, indescribable character of certain limited experiences you've had before only in your childhood. The feeling that no other childhood experience or other memories in your life have, that isn't even nostalgic, happy, or sad. It's just its own unique feel, like a wavelength of light. Just as we know the feel of the color red and dogs do not, there are yet many many more things that we do not know the feeling of, or can not describe well.