I hope all is well, present circumstances excluded. The answer to your question is broadly yes, you may have a cause of action against the administration of this tax based on the Constitutional grounds of Equal Protection under the 14th Amendment. In order to prevail on the merits, you would have to have the legislation that led to the administration of the tax ruled unconstitutional. Unfortunately, but truthfully, this could be an uphill battle as it is normally pretty difficult to get a judge to rule legislation as unconstitutional. Moreover, this is certainly something that would necessarily be appealed to the Michigan Supreme Court or US Supreme Court (depending on the filing), which means you would be in for a long and expensive fight.
As such, it would be best if you could organize a larger group of individuals that are affected by this law under the grounds of equal protection to share the costs of the suit...a class action type filing.
Based on what you have told me, it appears that you clearly have a case for unequal protection under the law. You would also not be the only one that has had their rights trampled upon. This appears to be clear age discrimination.
Another methodology to try to change the law is to lobby the Michigan legislature on the subject and convince them of the inequality that is involved therein. This too may be a long, tough road, but with enough people fighting along with you, it may be a road worth taking, even if you decide to file suit.
In short, it sounds as though you do have a cause of action for age discrimination in view of the 14th Amendment to the US Constitution. If at all possible, you should try to find others that have been adversely affected and form a class to file suit against the state on unconstitutional grounds.
Let me know if you have any other questions or comments.
Best wishes going forward!