Is it illegal for a 16 year old girl and a 18 year old boy to date?
There are no laws about dating.
Is it illegal for a 13 year old girl and a 17 year old boy to date in Missouri if the relationship is not sexual?
It is not illegal for a 13 year old and 17 year old to date/hang out together in Missouri. If it were every high school would have to close down. However, just because it is not illegal does not mean it cannot be prevented. As minor, a 13 year old girl can have restrictions placed on her by her parents or guardians.