This logic doesn't work for me either. I don't care if you are 50, if you live in my home, I have some say. I lived with my parents after 18. I paid rent, had a job, went to community college. Still had to follow their rules. Because it was their house. 

Even if skid moves out, into a house or apartment, there are still rules to follow. Some places won't let you paint or put nails in the walls, park on the street, have noise restrictions in place at certain times of the day, etc. If you have roommates, there are "rules". ALL living arrangements have rules. 

(And by rules, when we discuss skids, I assume we are actually talking about common decency/ courtesy.)

At 17 he would be a child who listens to you and obeys your rules because you are the parent and are legally responsible for him.  At 18 his status changes.  He is now an adult who listens to you and obeys your rules because you are providing his support and allowing him to live in your home.  In my home, if an 18 year old didn't follow the household rules, his status would change and he would no longer be an adult living in my home.

My house, my rules. 

However, I don't need to know where you are every minute any more, but I would still expect basic decency like home for dinner or not, in late or not. 

Well, how did he mean that remark? You don't have say over where he lives, who he marries, whether he can get a license, whether he stays in school, etc, but you do have control over what you offer him - a home, food, clothes, a phone, a car, etc.

HA! They are still KIDS in my house until they don't live there anymore! Don't like the rules, they are old enough to sign a lease and figure out life on their own. SS19 is special needs, so he doesn't really count but my Bios are perfectly aware how things are going to be once they turn 18 and are still in high school. 

I have also raised my kids to know from an early age (their dad has the same philosophy) that after graduation you have 3 choices. Live at home and work AND go to college. Go away to college and work. Or move out and work. SS19 once again is different, he is age adjusted to 12 but he does work. He does have chores. He does have tons of rules (like any other 12 yr old would). He just got grounded from video games. He isn't allowed to drive unless we are with him in the car. Etc. My kids to understand that he isn't typical and will probably never live on his own.

However, it is good for my kids to see that when SS tries to tell me that he is 18 (19 in 2 days) and an adult now, that I will smack him down so fast his head spins. I like to remind him that adults don't have to be reminded to shower daily- BTW, did you shower today??? He will stop off and take a shower after that and usually doesn't try it again for a few months. I can use shower, eating, using a stove, laundry, brushing teeth, driving a car... I have a solid 10 years of his non-adulting skills! Adults don't still live at home with mom and dad (or SM and Dad, Sdad and BM, whatever). Until they have their OWN place, and I am not talking giving $50 in rent a month to us, they will all be treated like children. 

What was the meaning behind that statement?

That at 18 your SS can do as he pleases within your home? Or that SS no longer has to follow a custody order and can come and go?

First, your home your rules. Age doesn't change that. The only thing that does change is that if SS doesn't follow the rules he is an adult therefore will be asked to leave.

If it's the latter regarding custody order and coming and going well that would be a hard NO for me. My home and I expect common courtesy. Which means I know when to expect a visitor, for lack of a better word.. No drop ins whenever you feel like it. No revolving, 24/7 door. 

My response to that:

You're right - I have no say over you anymore... I also no longer have the responsibility to feed you, clothe you and have you live here - so of course you can do what you like, since it won't be in this home. I don't foot the bill for freeloading adults. Buh-bye!

My status did change when I turned 18...I no longer had a status in the household. My dad told me to GTFO and get my own place. Granted, I had 2 years notice, since at 16 he started telling me the second I was 18 I was out the door. There was no "living at home getting everything paid for". Whether he was bluffing or not, I was out the door and had my own place the summer I graduated high school.

Does anyone remember hope chests? I had a hope chest for years prior to turning 18 that I put items in, and gifts at holidays were things for my hope chest- dishes, towels, etc. This was all in preparation for me to launch when I graduated. Those were the days when kids launched at 18.