Agreed on the second part, but on the first part you could just take the age in at sentencing. It worries me when you have a situation where law as written can be changed by the whim of a judge. What if it went the other way. Perp is 20, but because we’ve decided he’s really immature, we’re going to charge him as a juvenile.
I’d prefer the judge to have leeway on individual cases. In the grand scheme it makes for more justice and less of a legal system.
The only way a 20 yr old can be treated as other then an adult is to be determine to be insane or mentally incapable (IQ < 70 or so).