Put

enum Input {
  Text(String),
  Number(i32),
  Decimal(f64),
  ...
}

into the description may be better

I would recommend adding more sample tests, so that they look something like this:

test.assert_equals(is_adult(18), 'You\'re an adult')
test.assert_equals(is_adult(25), 'You\'re an adult')
test.assert_equals(is_adult(12), 'You\'re a minor')
test.assert_equals(is_adult(1), 'You\'re a minor')

Too few test cases - adding more, including edge cases, would make the kata more challenging. Right now it seems like an 8 kyu kata.

Still a complete duplicate without any originality.

There's already an abundance of "choose one of the possible values based on the input" katas.

I feel as though this kata is too simple, it took me only seconds and a line of code to complete. I understand why this could potentially be beneficial to new coders, but it may not be difficult for them either. I recommend having 'hello' change depending on the type of the variable you are passing in. This could be something like saying "Hello number " + str(x), for an integer and having a word to replace 'number' for a string. Anything like this could make the kata slightly more challenging, but I doubt it would make it more than a 7 kyu kata.

Also, no random tests

Not original, and pretty much a duplicate to this kata:

https://www.codewars.com/kata/grasshopper-debug-sayhello

EDIT: an exact duplicate: to

https://www.codewars.com/kata/thinkful-string-drills-hello-world