When they went off to college, you turned their rooms into your home office, gym, or (wo)man cave. Now they're back, maybe just for a pit stop on the way to real life, but they're not the same kids who left and you're all aware that even though the rules and roles have changed, it's not clear what the new ones are -- yours or theirs. And while it's tempting just to stick with the old ways of doing, being and behaving, it's not likely to be very good strategy in the long run, even if you can enforce it.
Coping with grown kids returning home is not only a relatively new challenge, it's a more prevalent one than ever, according to the most recent demographics. It's true not only in the U.S., but in most European countries as well; more young adults live with their parents than with each other or their romantic partners. While it may be a matter of economics for some , for many it's also an indication of the desire of both generations for each other's company, attention and support, emotional as well as financial. What's necessary for it to work is parents' realization that their home-agains aren't interested in the old status quo; they're not just seeking the comforts of being known and accepted for who they were but also acknowledgement of who they've become since they went away. They're probably not fully financially self-supporting, but they're still independent adults with the right to live their own lives. They want your understanding but not your advice; your interest but not your inquisitiveness; your acceptance of their choices and decisions, even if they're not the same ones you'd make if you were in their place
And that's my advice to post-parents -- remember that even if they're in your place, you're not in theirs, at least not given their life stage, their experiences, and all their recent history. They've grown up in a very different world than yours, and learned life lessons that have changed them in ways you may not notice. They face a very different world than you did at their age, and n even more different future. So do more listening than talking. Ask, don't tell. And make sure that before they leave again, you get to know the people they've become. It's a good start to the rest of your relationship with them.