Why do people keep talking about him when he was 12, what has that got to do with anything?
He was 12 when the Nazis came to power. Changes the conversation considerably. No one can stand here and say, with complete confidence, they'd be different from him. How much can you blame someone for being raised to believe terrible things, and then continuing on as a young adult?
It doesn't absolve him from his crimes, but we aren't talking about normal circumstances. This is clearly someone who, as an adult, changed his mind, and admitted wrong. It's 70 years later now. It's not yesterday. Maybe if he was older than 12, and had complete agency over his actions, we could talk about this differently. Until then, I think, it's not worth punishing him, again, after all these years.
This does nothing good, and no one can give a concrete reason as to why it would be good, without some vague use of the word "justice" or "lesson." How about something tangible? In every way, this sounds like, "We want to punish someone to make us feel better" rather than to actually accomplish anything. I don't think that is a good reason to lock someone up. That's a very serious thing to do. Prison shouldn't be the go to answer for anything ideological. Stripping others of their natural rights shouldn't come as easy as, "it will make everyone feel better."