Why is the legal adult age in America 18 and not 25?

Your brain may not be completely developed, but it is mostly developed. Can you imagine the strain on the foster system if people have to stay in until 25? Are you also suggesting that we extend the public school system until 25? If not, can you imagine having to obey your parents in everything after graduation? You wouldn't even be able to choose your own career if your parents had other ideas. You can't keep 15 year olds from having sex, but you think you're in any way going to realistically prevent it until age 25? You are now also delaying your adult learning period until 25. That means that instead of realistically being able to clean up your missteps by 25 or 30, you're pushing that back until 30 or 35.

It is considered the point at which most people are considered mature enough to function as adults.

You are correct in saying that biologically, some changes in the brain may persist past that time. However, this does not mean one does not attain a requisite level of maturity to function in independently in society before all biological criteria are met. There are many people at 18 who are independent, have jobs, relationships and so forth, and while they may still have much to learn, they do generally have enough maturity to function independently.

Your question is a good example of how science can be misapplied if one doesn't consider what its practical implications are. What you're doing here is prioritising laboratory and biological metrics over behavioral ones, when the issue at hand is one of behavior. To put it another way, even if one can demonstrate that biological changes in the brain are still going up to 25 or 30, most people have attained enough maturity to function as adults long before that age, so it is clear that the biological changes are not a good marker of one's actual behavioral maturity, which is the priority here. By 30 for instance, most people are independent of their parents, well on their way towards advancing their careers, and many are either married or preparing to get married and have children, and are adults in any meaningful sense of the term- so as far as adulthood is concerned, who cares what their brain looks like under the microscope? You're an adult because you act like an adult, not because the lab can't find any more changes in your brain.

Mistakes such as this are a common issue in medicine and are frequently discussed in medical school, especially these days when lab tests and scans are so common. It's very important there to make sure one is considering the patient's clinical situation, symptoms and history, not just what the lab results are telling you. For example, if you look at joints, pretty much everyone over the age of 65, if you scan their joints, will have signs of arthritis. However, the majority of people don't have any symptoms of arthritis even at these ages, so we don't treat them- we treat people if they have painful joints, not because the scan tells us to. Likewise, we consider people adults because they are mature, not because the lab tells us some biological changes are still ongoing.

I don't know but I agree with you.