Why does it take so long for us to form our first memory? My guess is it's because human memories are ideas and like all our ideas they depend on other ideas for their meaning.

However when we were young our ideas about the world were very different from what they are now: much simpler and with many falsehoods. So our earliest memories don't make sense to us and can't be recalled.

What we call our 'first memory' may just be the earliest thing we can recall and make sense of now.