My question is...
What is the boy is 16 and its the girl who is the oldest at 18? Is that against the law for her to be with him?

The age of consent in Michigan is 16.

The laws in Michigan apply equally to either gender. Within the laws, they use terms like "he or she", "a person". See below.

750.520e Criminal sexual conduct in the fourth degree;

1) A person is guilty of criminal sexual conduct in the fourth degree if he or she engages in sexual "contact" with another person and if any of the following circumstances exist:
(a) That other person is at least 13 years of age but less than 16 years of age, and the actor is 5 or more years older than that other person.
(2) Criminal sexual conduct in the fourth degree is a misdemeanor punishable by imprisonment for not more than 2 years or a fine of not more than $500.00, or both.

750.520a Definitions

(o) "Sexual contact" includes the intentional touching of the victim's or actor's intimate parts or the intentional touching of the clothing covering the immediate area of the victim's or actor's intimate parts, if that intentional touching can reasonably be construed as being for the purpose of sexual arousal or gratification.