I know age is common scale to measure 'maturity', yes, majority seems to fall in it fine. But the society change. Nowadays, 18 years old do not have the same mindset as 18 in 60s or 70s.

As for perfect crime. I should add that, nothing is perfect. The more precise would be. The crime's technology is AHEAD of forensic. so at THAT time, it is perfect crime. It will be crack when the correct technology is invented. In that instance, innocent people could be accuse, or the real culprit could be getting away what they done.

*sighs* Hard to justify justice. It is complex.