TL;DR: No, you do not replace every cell every seven years. Your body turns over roughly 330 billion cells a day, about 1 percent of its ~30 trillion cells, but this is dominated by blood and gut, which renew in days to months, not on any tidy seven-year clock. A few structures are never replaced at all: the neurons of the cerebral cortex, the crystallin proteins at the core of the eye lens, and tooth enamel last for life. The upshot for identity is that you are less like the matter you are made of and more like a self-maintaining pattern that the matter flows through, which is why a perfect copy of you would be a successor rather than a continuation.
Short answer: you are not, mostly, the stuff you are made of. Most of your matter flows through you and gets swapped out, the way a river swaps its water while staying the same river. But there is a catch that ruins the tidy version of this story, and once you see it the old puzzle of the Ship of Theseus stops being a curiosity and turns into a question philosophers still cannot agree on. Almost all of you is being rebuilt while you read this. A few parts, including most of your thinking brain, are never rebuilt at all.
Let me lay out what is actually known, and then where the certainty ends.
