Are parents capable of unlawful imprisonment?

AT 18 YOU CAN MOVE OUT MORON

Sorry, but if you live under their house, you live by their rules. If they are being abusive about it, then yes, it's unlawful. But if you got caught with some pot and your parents grounded you for example, stop being a brat and take your punishment.

That would be for a social services investigator to decide.

Yes but, not in your case. While you live in their house, you live under their rules and age 18 is not necessarily a magical number. In some states, the age of majority is 19 or 21. You cannot move out until you reach the age of majority in your state. IF in your state, the age is 18, then you can move out. In order to do that, you will need money. Do you have a part-time job? Save for an apartment. I find it hard to believe that they have imprisoned you when you are free to use the internet.

Capable, yes.
Of course, what do you mean by independent?

I would think so but if things were at that stage it would be wise of parents to allow feedom