Neuroplasticity
Your brain physically changes with what you do, at any age. Slower after childhood, but it never stops.
Every time you learn something, connections between brain cells get stronger, weaker, or new. Do a thing often and the wiring for it thickens. Stop, and it thins. The word for that is neuroplasticity, and for a long time people thought it ended in your twenties. It doesn't. Evidence: solid
What changes with age is the price. A child's brain rewires almost for free. An adult's rewires when he pays attention, makes mistakes, and keeps at it. Evidence: decent
Why it matters
It means "I'm just like this" is rarely true. It also means whatever you do most is what your brain is getting better at, including scrolling and worrying. Evidence: decent The brain doesn't know which habits you meant.