In your suggestion you were talking nonsense. And now you're talking nonsense as well. How on earth did you get 0 * 2 = 30???

Invalidating solutions is not a problem, users do not lose honor because of this, but rewriting a kata to change output format? Lol, good luck finding a user who's gonna waste their time on such a worthless idea.

Both father and son are aging simultaneously - that's why I thought it would be better to consider it in this kata

It may sound surprising to you, but if the user does not address this fact, (s)he won't be able to solve the kata.

And you're not suggesting to "consider the fact they're aging", you're asking to change the output from integer to string, which only makes this kata harder because you have to output the correct string i.e. consider the singular/plural use of word "year(s)" and choose the correct tense for the message (Past Simple, Future Simple). This would be harder than finding the answer itself, you know?

He already explained that. You'd be invalidating a -lot- of existing, valid solutions just becuase you want a output string.

Please, make good suggestions. Nobody's going to rewrite 7 versions of a kata because you'd like output to be slightly different.