How do we know the age of continental rocks if the crust is always changing?
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Some elements degrade themselves, overtime. It happens at a predictable rate, so by knowing how much it degraded, we can know its age.
Some minerals can contain radioactive elements like uranium, and *cannot* include their decay products like lead. And also the minerals are strong enough and chemically stable enough to survive millennia of rough and tumble after they’re formed in a cooling body of magma, even after it has eroded into sand.
Take such a crystal, and analyze the isotopes it contains. Slight oversimplification: you now know how much uranium has decayed into lead, so you can calculate how many half-lives have passed since it was formed.
> Zircons from Jack Hills in the Narryer Gneiss Terrane, Yilgarn Craton, Western Australia, have yielded U-Pb ages up to 4.404 billion years,[34] interpreted to be the age of crystallization, making them the oldest minerals so far dated on Earth.
https://en.wikipedia.org/wiki/Zircon#Radiometric_dating
There are some parts of the crust that haven’t changed, near the centres of the various continents.
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