Why is the definition of adulthood "finish maturing"? If anything that should be middle age, i.e. in the middle of adulthood, not the start of it.

By your definition as soon as someone becomes an adult they start degrading in mental ability. The peak of mental ability should match the middle of the productive years.

But in any case the maturity of the brain is not just biologically controlled - it's environmentally controlled. The brain of someone treated like an adult will be more mature than that of someone teated like a child.

The brain responds to stimulus, it doesn't really have a pre-programmed makeup. It has basic abilities, but that ones that are expressed are the ones that it needs/uses.