I'm a high school teahcer. I have been closely acquainted with thousands of teenagers. Nothing is out of the ordinary. An 18 year old boy or girl should be able to handle the day to day functions of living, thoough.
Preparing simple meals, cleaning up, doing the laundry, and other basic household chores (provided they have been taught) --all to be expected, but with some 18 year olds, not without a fight.
The most common house rules I hear from parents of these "young adults" are these (after high school graduation).
1) You live at home and are not going to school, you pay rent, and your expenses (your share of utilities, groceries, car expenses, clothing, entertainment, etc.)
2) You follow the house rules (about chores, family time, cleanliness, noise, guests, staying out all night, etc.)
3) Once you move out, you can move back home once, and the above rules apply.
However, in truly unfunctional families (most frequently one parent/one child), I find the adult has done everything for the child, all the way up to adulthood, and the teenager knows nothing about the basic functioning of a household, meals, laundry, etc. When the parent finally realizes that they have a useless schmoo on their hands, the schmoo gets very angry about being called to task.
One of the more articulate schmoos told me that for 18 years doing nothing was good enough for her mom, her mom just did everything and still loved her, then all of a sudden, she was no longer good enough, it was very confusing and hurtful for her. She had a gentle, calm personality, so the battles weren't raging, but she was still upset, thrown off balance, a little angry and afraid because her mom just changed all the rules, overnight.
Eventually they all survive. No one has ever died having to do their household chores (at least not in this part of California).
And some teenagers operate on a different clock than the rest of the world. Supposedly there is research on this, but I haven't seen it myself.
Good Luck. Parents tell me 19 is worse than 18...
Shoshana