^Excuse me, but how do you see the solution from the author?

Without a generally agreed upon difinition of teens as over 12, 10-19 as used follows nicely from the description.

Closing.

There is no definition of "teen" in kata description. This term differs from country to country and by analogy with other groups, it is logical to assume that in this case it is from 10 to 19 years, inclusive.
In addition, after solving the kata, you can see the solution of the author, from which it also follows that 10 <= 'teens' <20.

#Issue#

Description asking for teen range but tests are mistakenly passing ages 10,11,12. Either, or needs to be corrected. (Tested with JavaScript)

This is not a correct answer as it doesn't return false for ages 10,11 and 12. The question asks for teens and up.

This is not a correct answer as it doesn't return false for ages 10,11 and 12. The question asks for teens and up.

This is not a correct answer as it doesn't return false for ages 10,11 and 12. The question asks for teens and up.

This is not a correct answer as it doesn't return false for ages 10,11 and 12. The question asks for teens and up.

This is not a correct answer as it doesn't return false for ages 10,11 and 12. The question asks for teens and up.

This is not a correct answer as it doesn't return false for ages 10,11 and 12. The question asks for teens and up.

I didn't know we weren't supposed to use for...in with arrays. Just googled it and understood. Thank you.

The problem is you're using for...in with an array. Add console.logs to see why your code is failing.