It's hard to say anything about mental maturity based on age IMO.
I have seen 16 yo who are smarter and more mature than some 20 yo. That's not to say that the 16 yo are mature adults, but rather that even 20+ yo can be immature and even 16 yo can be mature. But it's for sure less likely to be the case for a 16 yo.
Not fast tracked, efficient. A proper process, and as long as it takes to be convinced beyond reasonable doubt, just a system where the proper process doesn't take a billion years and a billion dollars.