The hostels you are talking about are likely HI hostels. They sometimes have an age cap on guests at 26, though this is really only enforced in certain parts of Germany I think. I think the reason is simply to be difficult (and maybe something about giving youth a cheap place to stay). You shouldn’t run into many problems because of this, as private hostels almost never have age restrictions. Besides, private hostels are usually nicer.

I stay in German HI hostels a lot.  Once upon a time, 27-year-olds were kept out of German HI hostels; this practice persisted in a few areas of Germany until recently, but it is NOT a practice now.  HI hostels in Germany (same hostels as “DJH”, the German acronym for German Hostel Association) will admit anyone of any age.    The only “restriction” is that in one of the German states – Bavaria – those under 26 receive booking priority.  It isn’t exactly clear what this means, so if you want to book a Bavarian HI hostel, contact them anyway and see what they say.

There are about 600 HI hostels all over Germany – they outnumber private hostels by far.  You’ll find them in big cities and in little villages along hiking trails.  Most German HI hostels have very good or excellent facilities.  They are generally very clean, have 100 or more beds in 4-6 bed rooms with private baths, and offer a generous breakfast, included in the price.  A handful of them are situated in former castles.  Don’t scratch them off your list. 

My only caution would be that the reason German HI hostels are so nice overall is that they are heavily patronized by school groups on week-long outings – lots of taxpayer cash supports them – but this means that you have to share the hostel with a bunch of nutty prepubescents during the school year.  If you use the HI hostels after about the first or second week in July, you are generally pretty safe and will sometimes have the place to yourself.

And Sir Tobys, in Prague, won’t let you stay in the dorms if you’re over 40.  You won’t have to worry about that for awhile.

I did not know that MB, I stayed at Sir Toby’s before I reached the tender age of 40, but I still highly recommend that place

Yep.  Per the email they sent me.

Thanks for your responses everybody.