^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.

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