It's a beautiful idea, when you think about it: your body replaces all of its cells every 7 years, so in that time you've left the old you behind and become a completely different person. Unfortunately, it's just not true. No one knows where the 7-year myth came from, but even if it was a rough average of every cell's lifespan, it would still be wrong. Some parts of you remain for your entire life. The cells on the inner lens of your eye have been there since you were a fetus. Your tooth enamel is never replaced, and neither are the neurons in your cerebral cortex. Luckily, neurons in other parts of your brain do regrow. As do other parts of your body: colon cells, for instance, refresh after only four days, and you get an entirely new skin every 2-3 weeks. White blood cells take more than a year to regenerate, and it takes an average of 10 years to grow an entirely new skeleton.
If you'd like to learn more about human biology, check out "Sapiens: A Brief History of Humankind" by Yuval Noah Harari. We handpick reading recommendations we think you may like. If you choose to make a purchase through that link, Curiosity will get a share of the sale.