According to Kansas Law the age of consent for someone is 16 years of age.
So if the 16 year old consented to sex with the 18 year old then it's usually legal.
But if I were that 18 year old I certainly wouldn't want to get into that hot water should that 16 year old tried to claim rape should the relationship go sour.
If the 16 year old tried to claim in court that the 18 year old had sex with them without their consent then that could cause the 18 year old to go to jail or prison and be labeled as a sex offender for the rest of their life.
So while it may be legal for a 18 year old to have sex with a 16 year old in Kansas it would not be a good idea.
If you really love that 16 year old then the 18 year old can easily wait for another few years when the 16 year old turns 18 which would make the sex more legal and prevent legal troubles.