For a long while, Amazon would let you share some of your Prime perks with four other people of any age -- handy if your roommates need to get an order in a hurry. However, it looks like the internet shopping pioneer is clamping down. A quiet rule change on July 31st has limited Prime sharing to one other adult as part of an "Amazon Household." Moreover, that other grown-up has to use the original member's credit card (update: not really true, see below). That's fine if you're sharing it with a significant other, but lousy for anyone else.