Hostelling International is the world’s largest accommodation provider. Most youth hostels accept reservations. Almost none have age limits.
IYHF(International Youth Hostel Federation) publishes two annual directories of Hostelling International hostels: Volume 1: Europe and the Mediterranean – Volume 2: Africa, Americas, Asia and the Pacific.

Private Hostels & Backpackers
Hostels.com is the most complete for hostels worldwide, with also many private and independent hostels.
Hostelworld.comis a web booking service for private hostels. It lists fewer hostels than hostels.com, but has a wider range of budget accommodation, campsites, etc…