Let's start with minors.
This is mainly a question of the age of criminal responsibility in law.
People who have reached the age of 16 commit crimes, that is, they should bear criminal responsibility, which is the age of full criminal responsibility.
According to your question, then 16 is an adult in criminal terms.
2\ About A who was under the age of 18 when he committed the crime, he was sentenced to 3 years' imprisonment and was over the age of 18 when he was released from prison. Is A a released minor?
I'm afraid this is a question of understanding the text.
Since he was over 18 years old when he was released from prison, this person was released from prison at 18 years old.
If you think 18 is an adult, then this person was an adult when he got out of prison.
So this person is an adult released from prison at this time.
In any case, it has nothing to do with the age of the trial time.
By the way, do you want to investigate \ how to investigate the age is the age at the time of crime, not the age at the time of trial.