Huge assumption to make that the guy will want to find a normal job and be a good member of society someday.
Because someone who commits a crime (or multiple crimes) when he is 16/17 is predestined to become a criminal for his whole life? This would be a way bigger assumption.
You actually don't need a lot of criminal energy to commit computer crimes from your children room. Even if you can commit severe crimes with the computer - the hurdle is a lot lower than for example to rob someone - as everything is virtual.
And even swatting (which is NOT just a prank) - can be seen as a prank (and probably often is - that's where the problem lies).
Kids (and even 18, 21 year old or older) are often stupid and don't act responsible. This isn't really uncommon. There is no reason why one shouldn't assume that he grows up eventually.
He can go to university (it is free there) and get a decent job. Maybe he can't become a lawyer anymore (don't know in germany his record would get deleted, I think), but I don't see why he is doomed to become a criminal for his whole life.