Criminal Sexual Conduct in the 3rd degree occurs when "a person engages in sexual penetration with another person and if any of the following circumstances exists:
(a) that other person is at least 13 years of age and under 16 years of age.
(2) Criminal sexual conduct in the third degree is a a felony punishable by imprisonment for not more than 15 years. MCL 750.520d
It does not matter if the actor was only 16. Whether he goes to prison depends on if he is charged as an adult or a juvenile, and if an adult the sentencing guidelines, but if he is charged as an adult prison is a real possibility.