Here's why:

* Formation Process: The Earth formed from a swirling cloud of dust and gas. As this cloud collapsed under gravity, it heated up and eventually melted. The heaviest elements, like iron and nickel, sank to the center, forming the core.

* Age: This process happened billions of years ago, making the inner core the oldest layer.

* Continuous Formation: While the core formed early on, it continues to grow as Earth's mantle slowly cools and releases heavier elements.

So, while the Earth's crust is constantly being renewed through tectonic activity, the inner core has been there since the beginning.