Providing the relationship is non-sexual it should be OK.
No! how is it wrong to like a eighteen year-old??
Yes
Yes. if you are 18 you can be prosecuted.
Yes.
Not if it is done with sexual intention.
Yes
Of course it is, it's called molestation and sexual abuse.
It is not illegal for siblings to share a room. Age has nothing to do with it.
Date? Yes. Have a physical relationship with? No.
There are no laws about dating. Not a good idea, because there is only one thing an 18 year old is looking for and that would be illegal.
The plural form of thirteen year-old is thirteen year-olds.
There are no laws about dating. However, any sort of physical relationship will be in violation of the law. And lets face it, the only reason an 18 year old would date a 13 year old is for sex and that is illegal.