I know people who died as brats.
Age doesn't define maturity any more than grades measure intelligence.
Age does not necessarily define MATURITY, but it DOES define IMMATURITY. Believe it or not, young people actually aren't capable of being mature. The natural, psychological things they will go through in their teens and their childhood makes it mentally and physically impossible for them to ACTUALLY be in any way mature. Don't mistake good grammar and use of language for maturity.
That said, Adults are the same. Some adults just haven't developed mentally yet, and are still immature.
one is defined in terms of the other
Nope, how boring if it did.
One would hope as we get older and collect more experiences that we would modify our behavior and act more mature. The learning curve is suppose to support this theory.
In reality some people just don't have the IQ to be able to learn so allowances must be made.
Some people learn bad behavior because of the attention it brings them, like an audience laughing at mad actor mad gallagher (the one that smashes fruit on stage).
It's taking it's sweet time to kick.