If a child can be tried as an adult then could the opposite be true?

No, but an adult can be found mentally incompetent to stand trial.

Damn. I think @cookieman nailed it.

I had a pretty good answer lined up, but, hey. Call it.

I think @cookieman pretty much said what I want to say…

It helps that my wife went to law school.

My wife would vote emphatically yes!

The argument that an adult should be tried in juvenile court (for a crime that he may have allegedly committed as a juvenile) was advanced by the defense / appeals team for Michael Skakel in the appeal of his conviction (in 2002) for the 1975 murder of Martha Moxley, when Skakel was a fifteen-year-old. The appeal has not been upheld, but it was an interesting attempt.

When mental incompetency becomes a factor, it’s almost seen in the same light as a minor. except where punishment is involved (probation, general incarceration, etc.).

In Germany, the ‘Jugendstrafrecht’ can be applied to adults between 18 and 21 years old, if their immaturity is certified by the ‘Jugendgerichtshilfe’.

When mental incompetency becomes a factor, it’s almost seen in the same light as a minor. except where punishment is involved its depend on the law and judgment.