Postgres Join that allows for an empty table

I have the following schema, and I'm having a lot of trouble joining these tables properly. I'm trying to get all folders that don't have a file updated in the last 10 mins in them. But I want to include any folders that don't have any files.

create table folders {
  id              int
  table_name      string
};

create table files {
  id              int,
  folder_id       int,
  file_name       string,
  last_updated    timestamp with time zone
};

My naive attempt so far is something like the following (this is a re-creation from the CLI):

Select t.folder_name from folders t join files f on (f.folder_id = t.id) where last_updated < 15m group by t.id

I can get either all folders to show up OR the latest file for each, but not both.

Any help would be greatly appreciated