you son/daughter is 16 and the person s/he is seeing is 18. i don't see a problem with this. i can understand that you still see them as your little girl/boy but s/he is stating to grow up and i know its hard but you are going to have to accept it.
its a 2 year age gap its not like their seing a 20year old so i wouldn't try to stop her. try and be understanding. i think the age 16 is old enough for your son/daughter to make his/her own dessision. aslong as you make sure s/he is sexually educated and tell her to be carefull and take it slow everything will be fine. if the relationship doesent work out s/he will learn a lesson from it. you have to start somewhere.