not a good idea but not illegal either.
Is it legal for a person that is 18 (born in 1998) to engage in sexual contact with a person that is 17 (born in 1999) in the state of Idaho?
It is not illegal. The law is if a person is 18 or older they cannot have sexual contact with someone under the age of 16. And no one who is 5 years or more older than someone who is 16 or 17 can have sexual contact with them.