Because young adults are going to have an increasingly-difficult time gaining and keeping work, and paying rent or for a mortgage, it is becoming more common for them to stay living at home for longer. I think, because this pattern is becoming more common, families will start planning for it as an expected stage of life. We'll build our houses to handle a few adults in transition to independence, rather than planning houses for young families that assume you'll be out the door when you hit 18.
Mokalus of Borg
PS - It's going to be a subtle difference.
PPS - Or no difference at all, depending on who you ask.