It is indeed by hostel, so check with each before you book. Regarding the previously mentioned age restriction of under 28 at the Bavarian hostels, I've seen that just a few months ago. That just means that the under 28s will get priority. If they have bunks available, they'll be happy to accomodate if you're over 28.
The reason for this is that in Germnany, where hostelling started by the way, hostels cater to school groups and in Germany, school groups take lots of field trips. I stayed at a hostel in Germany in the Hostelling International network, that had the age restriction, but I called them and they did have one available bunk room, which I got to myself. The hostel was filled though with high schoolers and surprisingly, it wasn't bad, they were all pretty well behaved.