Who are you?
Consider two foetuses, F1 and F2. F1 was conceived before F2, but was delivered after F2. Of the two, which one is older?
It would be F2 as it was delivered before F1, even though the latter was conceived earlier. As things stand, one's existence is recognized only when one gets out of their mother's womb. Why not recognize the existence of a life from the day it was conceived, if your birth date is truly the date of your origin. Is the birthday that you are celebrating, really your birthday?
http://www.whenwasiconceived.com/ : This website lets you to find the approximate date when your mother got conceived you, what song was topping the charts in the US and the movie that got released at that time. Give it a try.
That brings us to the question what is 'you'? If you consider from a physical standpoint, the cells which are building blocks of our body, get replaced constantly and with passage of time, the cells which used to be you, are no longer you. Now some new cells are you. You are a new person every once in a while.
Also 90% of our body is made up of microbiomes that have entered via our various openings of our body, enabling us to do various activities like digesting, immunity, controlling stress levels etc. That leaves only 10% of what you perceive as you, actually you.
Are you what your memories are, the ones which shape who you are? Babies don't make memories until they are 2 or 3. Is the baby you, not you? Should we consider people dead, if they lose their memory? I had a grandfather who passed away, late last year. He didn't have a clue as to who he was for about a year, until he physically died. What should have been his status?
Moreover the memories we make, tend to get less accurate each time we recall them. We always look back and say "Those were the times", when we reminisce about our childhood. There is this halo effect, where our brain flushes out all the unpleasant details, which would have given a less romanticised view of the whole thing. Our memory doesn't work like that of a computer. Our memory is stored in all parts of the brain. When it is triggered, it is a chain reaction of the neurons and the whole memory is recreated reflecting our current thoughts. It is always getting altered.
So, who are we? I don't know, let us ponder together.