Well, I am not a lawyer so I don’t know if that can really happen, but you are supposed to be judged by the law that applied to you when you committed the crime, not any different scenario that could be applied to your case in the future (nulla poena sine lege, non-retroactivity of criminal law).

Consider, for example, something that didn’t use to be a crime. For example, buying alcoholic drinks. If now they ban alcohol, they cannot start prosecuting people who bought alcohol when it was legal. Even if they announce they will ban buying alcohol, they cannot wait for the law to come into effect to start prosecuting people who bought it while the law was being written and knew it was going to be banned, because it was not banned yet when they bought it. This is not the same case, but it’s similar.

What matters is the law that applied to you when you committed the crime, not when you are being judged.

But, as said, I’m not a lawyer.

Being underage at the time you commit a crime is not a guarantee you won’t be tried as an adult. They can decide on a case by case basis already, so it was likely legal even if they had been tried at 16.

Is what jurisdiction is that legal!?

Generally questions like these vary by jurisdiction. Some allow trying a minor as an adult, like others have pulled out of their ass. Some don’t, some might not distinguish or don’t bother with a right to trial of any kind.

It’s a pretty typical principle that you try someone as they and the laws were at the time of the offence, though.

How is it legal? The law makes charging a minor as an adult legal.

Do they?

Surely if the person is 16 at the time of the offence and the offence is bad enough they will try them as an adult not wait until they are an adult.