Good question. Perhaps a more in-depth analysis of the person would be required to understand why they are not passing the test.

What happens when X or Y minority group claims the test is biased against them?

This is an existing pattern and I would defer to the legal system to mirror the existing processes. This pattern is replete throughout all intersections of society and government.

Also your claim is that an intelligent person who could perform well on an examination wouldn't also make risky sexual decisions when they are young?

I never said intelligent. I said "if a person is mentally competent to make legal, financial, contractual and overall life decisions". These things are not directly tied to intelligence alone and intelligence can be interpreted to mean many different things. The test would be written and reviewed yearly by clinical psychologists, biologists and lawyers. The current process for this that already exists depends entirely on a judge which is just a more experienced lawyer and a single persons judgement call that may be biased by their personal beliefs. I believe my suggestion would be a vast improvement over the existing system and would make it easier for kids put into precarious situations to move forward in life rather than sitting in emergency foster care at the mercy of strangers if they don't actually require it.

I personally know of a few people including a close friend that went through this process. She was declared an adult at age 14 and was appointed guardianship over her brother as the legal guardian with full custody. She handled this perfectly but all of this was the decision of a single judge. This is of course not the same as legally being at the age of majority/consent. In my method, there would be certifications for different aspects of responsibility much like endorsements for a car or aircraft license. These would be endorsements on their state ID.