As far as i know, people are still allowed to change the national team, if they haven't played any international matches for the A-team. It doesn't matter if someone played 100 games for the German (or any other team) U21 U 19 etc, as long as he hasn't made any games for the A team, he can still switch teams and play for a different country once grown up! I'm not sure if that's what you meant with German past, but that's how it goes.