our's are'nt 18 yet but with one turning 15 we've talked to her about it already and it's basicly OUR HOUSE, OUR RULES. it would apply if the household member were 8, 18 or 80. we don't choose to have "tenants" in our home though if one of our family members is an adult they are expected to contribute financialy. the exception would be (as they are witnessing with dear old dad currently) if they are in an educational program (college or as will be in both the kid's cases when they turn 18 still finishing highschool) with an established goal (sorry-no "professional students" that keep major hopping until their mid 30's

). if an adult were unemployed due to no fault of their own (health, disability, the current job market-though dh and i both believe that just because a job is'nt available in one's desired line of work that does'nt excuse them from making an effort to find employment such to provide income for their needs) that's a different matter.
we are hopefull that we can offer our children the same opportunity afforded me when i became an adult-lodging, meals, items of personal need (hygene, laundry products...), auto and health insurance-as well as some financial assistance should they choose to attend college. if they choose not to attend college they will be expected to secure employment and contribute at a set rate (fair market value) to the household. in either case the house rules stand and if they don't choose to follow them they can secure and pay for their own house and set their own rules (and dd is already at nearly age 15 aware of the impact of losing her legal dependant/student status as far as our health insurance goes-a MAJOR factor for her given a pre-existing medical condition that could create for her a tremendous financial hardship were she even able to find insurance outside an employer based group plan).
i'll share one thing that was edifying for our dd (did'nt set out to teach her this it just happened)-we sold our prior home and moved into a rental home before we purchased our current. dd happened to see the rental agreement and asked if she could look at it. she read through the pages and pages and was floored that "someone who a person is paying THAT MUCH MONEY to could have so many rules they THINK the renter will follow". dh and i quickly corrected her saying "no, the landlord does'nt 'THINK' the renter will follow the rules-they KNOW and EXPECT no less than that they will be followed-the renter is an adult, an adult entering into a binding contract where they are signing their name at the bottom to a statement that says not only have they read the rules but they aknowledge that they understand, accept and WILL follow the rules-and failing to do can result in eviction, monetary damages and in some cases criminal prosecution". dd was rather speechless-we ended the conversation with "being an adult is much more than just having spent 18 years on this planet-and you would be better off learning now that you have a WHOLE LOT MORE to learn about being an adult before it's thrust upon you just by virtue of a calendar date rolling around".