The exact number is a little controversial, but there's pretty universal agreement that most of your cells wear out, die, and are replaced within your life time

The exact number is a little controversial, but there's pretty universal agreement that most of your cells wear out, die, and are replaced within your life time. I didn't elaborate, because though it's useful for illustration, my point doesn't really depend on it - even if we had the same cells throughout our lives, we could swap them out one by one and still be us.