There are no laws about dating in the US. There are laws regarding sexual contact and even that shouldn't be an issue in most states.
Yes it's legal
No, it's not.
No..but yes
no
Yes, it is legal.
sure! why not?! you can date who ever you want!
It is legal to date a 21 year old man if you are a 17 year old girl. It is not legal to engage in certain activities with a 21 year old man as he could be prosecuted.
Yes
There are no laws against it.
yes.
yes it is.
No, the girl must be 18 years old.