"You are not young, either."
Translation:Ti sem vagytok fiatalok.
It is wrong.
Te sem vagy fiatal. is a correct version. sincs only appears in the third person, and now we talk about the second person.
But, in this case, even for the third person, we wouldn't use "sincs".
Ő sem fiatal. He is not young either.
Or trying the formal you (grammatically behaves like the third person ) Ön sem fiatal. You are not young either.