"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.
Does this sentence imply that they didn't get old together, or means plainly that they intended and we don't know the rest?