People talk about "old" coins, and "aging" coins, to avoid a fee, but what is the actual meaning of "old"? How long do they have to sit, is it a coupe blocks, a couple days, a couple weeks?
It's based on the number of blocks. I'm not sure about the actual number but I think it's on the order of weeks. The reason that age matters is to prevent relay spam where someone just transfers one satoshi back and forth between two addresses without a fee.