Dear Landlord loathing,
Well, first things first, I assume you have a lease that you signed when you moved in. Whatever that lease says, thems the rules, baby. As a tenant you have rights; legally, your landlords can't change the terms of your lease until it's up.
However, you're obviously a considerate person and don't want to have a strained relationship with your landlords/neighbors. It is weird that for months, your noise was tolerable and, all of a sudden, it's out of control (in their minds). I wonder, was this the first time you'd entertained guests so late? Consider that you may not have been as quiet as you thought you were being.
Even so, this was an overreaction on their part, especially since this was (seemingly) the first time you'd disturbed them. Unfortunately, your age difference probably doesn't help foster tolerance as they are less likely to need your understanding during their late-night parties (unless they're night owl dinner-party hosts themselves).
Here's what I think you should do: bake/buy some cookies and write a short note that says, "Sorry for disturbing you the other night." You want them on your side and since this is the first time they've reacted this way, give them the benefit of the doubt (even though they didn't give it to you, be the bigger person). Maybe the whole thing will blow over; it's certainly worth a try.
Continue to abide by the terms of your original lease and if you still run into problems, be really honest (without getting defensive) and tell them exactly what you told me: you love living there, you've put a lot of time into fixing it up, you always pay on time and you really want to get along and be a considerate neighbor but you also want to have guests sometimes as per your original agreement.
If they're dead set on a quiet mouse then you may be looking for a new place when your lease is up. But would you have wanted to keep living there anyway? I think not.