Is it sexual assault if an 18 year old had sexual intercourse with a 16 year old?

It depends entirely on the state: the rules for sexual assault and age of consent are set on a state-by-state basis. In certain states, if an adult (18 or more) has sex with a minor, it is a form of sexual assault; other states set a lower age of consent, so you don't to be over 18 to be considered to have given consent to sexual intercourse, and/or have an exception to the usual rule, such that if there is a narrow-enough age gap (e.g. 2 years) between the two people, there is no assault even if one is underage. You need to look at the "penal code" for your state, to see what rules apply: look under "crimes against the person" to find the different categories of sexual assault. Or repost you question, indicating what state you are writing from.