purely guessing (I'm not at the office, so I cannot look into any CRS), I'd say high season will end sometime beginning to mid September; the last set of Summer holidays - Bayern - ends on 12. September, with Baden Württemberg ending just two days earlier, so you've still got quite a few people using the last one or two weeks for a trip out of the country...
But keep in mind that all people that are not bound by the holidays usually want to leave within one or two days after the high season is over - which can sometimes lead to cheap high-season-fares being better than the expensive low-season-fares that you'd be getting, because the flights are already filling up again.
By the way - is the age-group you've got in your profile correct (21-25)? If yes, then you're still eligable for most youth fares, because most of them are for passengers "below 26 years" - LH
being exceptions that come to my mind right now, because they only accept "below 25 years" as youths; anyone above that would need to be a student with ISIC card.
Personally, I'd always use a website (that is, if you want to book over the internet) that's not an airline website - but use something that looks though several airlines; if you find something good there, you can always go on to check the airline's own website to see if they've got an even better offer - sometimes they do, sometimes they don't.
Personally, I usually check through things like Expedia or Opodo, as well as the website of my own employer - but I usually end up booking flights for myself through the CRS at the office...
Feel free to eMail (or IM
) me if you need any further hints.