@Deirdre110 Are you renting a private room in a shared home, or a self-contained unit (Entire Home listing) in which the hosts are having their party in a separate household?
If the former is the case, you would have a pretty strong basis to have the remainder of your stay refunded directly from Airbnb due to a high Covid risk scenario. You absolutely shouldn't be subjected to a house full of people sharing common areas you must traverse, if the listing wasn't advertised as a party venue.
If the latter is the case - you have your own entrance and share no indoor common areas with the host - you'll most likely have to negotiate the solution. If you want to leave early for any reason, follow the procedure detailed here to request a change of the checkout date: https://www.airbnb.com/help/article/913/how-do-i-change-a-reservation-for-a-place-to-stay
The host would have to approve any changes made here in order for you to receive any kind of partial refund for the unused nights. If they decline, you could escalate the case to Airbnb, but I can't say for certain that whichever outsourced operator you get would consider one night of noise on your first night to be a Travel Issue that encompasses your entire booking.
Re: the "20 year old with no adult supervision" - a 20 year old is legally an adult in Switzerland, so this is not a point worth advancing.