Your body is constantly tearing itself down and building itself back up. Different cells get replaced at very different rates. Your skin cells get replaced every 4 weeks. Stomach lining cells every 4-5 days. Red blood cells every 4 months. Bone cells every 10 years.
Add it all up and many parts of your body do get rebuilt over a stretch of years. But the often-repeated “7-year cycle” is a rough average that hides big differences - some tissues turn over in weeks, others take a decade, and a few barely change at all. You’re more like a continuously-restored building with some permanent fixtures.
There are real exceptions. The neurons in your brain mostly stay with you for life. The lenses in your eyes don’t replace either. Egg cells in females are present from birth. And the enamel on your teeth - the hardest stuff in your body - never gets renewed at all once it’s formed.
