"The couple would have gotten old together."
Translation:Paret ville ha blitt gammelt sammen.
It's the same as in English, where a couple is a singular grammatically, but because it consists of two people you may sometimes treat it as a plural:
The couple next door are fighting again.
The couple next door is getting married next week.
You could say that using the singular puts a focus on the entity, while using the plural puts a focus on them being two different people, but the Norwegian sentence above is so focused on unity already that I don't think it adds any such nuance. While I'd favour the singular in this case, I can't say that the plural sounds wrong to me.
The correct answer to the exercise was "gamle". In Norwegian there are three genuses, and if you talk about two persons, you could call them "a couple". In Norwegian that word, "et par", is in the genus "intetkjønn", or the neutral genus. That means that you add a T to the adjective. Example: Huset er gammelt (neutral), mannen er gammel (masculine), kona er gammel (feminine).