In Michigan, possession of marijuana is a misdemeanor. It does not matter if you are a minor or an adult. Age is irrelevant when it comes to marijuana possession or use in Michigan. Only the act itself is punished, with a fine up to $2000 or up to a year in jail for possession, and a fine of $100 and 90 days in jail for smoking marijuana in any location. Judges have considerable discretion in sentencing.
Everything I can find doesn’t differentiate between penalties for minors and adults. However, I did discover that there are much harsher penalties for marijuana related charges than alcohol related charges when it comes to minors. Most possession charges are all seen as misdemeanors, except for some cases where the possession charge is acquired in a public park. Sale and cultivation charges are basically all felonies though.
The marijuana laws for adults and minors are different. An adult can purchase medical marijuana after they receive a Physician Certification. A minor can, but they would need 2 Physician Certificates by two different doctors. All patients must register with the Michigan Department of Licensing and Regulatory Affairs, Medical Marijuana Registry Program.