There are so many ways to respond to this interesting question, thanks for asking it!
You could be your body, or some collection of material structures in your body. In this case you're mostly less than six months old, as your metabolic processes are continually replacing your tissues with new molecules built from the food you've taken in. The old you has broken down and been excreted.
You could be your mind. Your personality. Your opinions. Your thoughts and feelings.
You could be your commitments - your word. You could literally speak who you are as your free creation.
You could be a portal between the (source / ground of being / God / Buddha nature) and the material world. Would you then be the same being or a different being from others?
You could be what you contribute. Your work, your nurturing of the important people in your life. The love you give.
You could be a mystery.
You could be.