Is it illegal for a 16 year old and an 18 year old to have sex in the state of kansas?
Or would the 18 year old run the risk of getting into any sort of trouble?

In the State of Kansas, the age of consent is 16 (minor).
The age of majority is 18 (adult).

In the State of Kansas, a 16 or 17 year old is legally capable of consenting to have sexual intercourse; however, at age 16 (still a minor) legal authority falls to the parents. If a minor age 16 engages in sexual activity with an adult 18 years of age or older, the adult may not be charged with a sexual offense; however, a parent may exercise their parental and protective rights to prevent any adult from pursuing a sexual relationship with their minor child through available legal channels.