Is it illegal for a 18 year old boy to have a relationship with a 28 year old girl?
No at 18 you are an adult and so is the 28 year old. Frankly, I
can't imagine why a 28 year old WOMAN would want to have a
relationship with an 18 year old boy. The age difference is too big
when you are 18 and the other person is 28.