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It really depends what the expectation is here.

On face value this wish is not different then “i want to be a doctor, marine biologist, astronaut” These things require years of professional expertise.

But does the kid really want to become an degree holding expert in a field… or do they just want to feel like what they think the job entails.

Its definitely incorrect that being a “hacker” is illegal. In many interpretations a hacker is simply someone who uses technology creatively in sometimes unintended ways. But i am assuming they mean computer hacker specifically.

My suggestion would be to start with giving the kid a laptop with linux and to follow some guides on how to use the terminal, you learn a lot about how computers work that way. There are also apps to emulate a linux terminal on a phone/tablet which can be complementary to the laptop to experiment with a small network of devices.

A starter project would be to build a bootable usb that you can plug in any laptop/pc usb port. They could use that as their personal “hacker os” that can be kept safe seperatly from their device. With this technically any wild pc with an unlocked bios can be temporarily be turned in your pc where you are admin with your tools and files where you are admin.

Another cool project they could learn is how to setup a remote connection to acces a terminal on a pc on another network (using ssh-key (secure), not passwords (insecure) it does mean someone needs to setup that pc with an open port but the first time i tunnelend into my own network this way it definitely made me feel like a hacker. And a “real hacker” would need to know those basics first anyway.

You might also want to check sdf.org they provide free unix shells and guides for all kinds of stuff.