Several years ago, Reddit changed its archive system to a subreddit-by-subreddit opt-in system. So if posts archive after six months or not is up to the owners of each individual subreddit. That's why some posts less than a year old are archived and ones more than a decade old aren't.

Also, yes, I know you asked this over a year ago and probably already knew everything I just said. But people are still asking this question, so why not just continue the madness?