Interestingly, my eldest two both seemed to have a year or two when they knew their right and left instinctively, as you describe, then I suspect formal education got in the way and turned it into something you had to think about, and they've not yet got back that easy assurance. Number three is still perfectly reliable, at just four, and unlocks the door for her nearly-eight year old sister who gets worked up about which is the right way...