Maybe in USA. In Europe some countries even have special sentences for young adults (older than 18 less than 21). Here's for Germany:
"The maximum penalty for any crime committed by a person under 18 (or a young adult under 21 who is treated as a juvenile) is 10 years"
(Considering that people become more or less aware about their responsibilities and consequences of their actions at about 15 yo, he is now 4 times the responsible age he was at 16.)
If you believe this, its because you aren't looking. There's tons of explanations online of how it could be possible. Go look at /r/chess.
Does that seem fair or reasonable to you? Sure no man can be as good as Magnus, he is clearly cheating in some clear undetected way, right?
For those two reasons, I think that any such "pretty adamant" statement can be discounted.
Your comparison with murder is ridiculous. First of all, teenage murderers are regularly sentenced to life in prison. A murderer is deprived of fundamental human liberties--Niemann is deprived of being able to compete at the highest competitive level in a tabletop board game, without suspicion.