You don't usually need to book, that's the beauty of them. Although depending on the season and location, sometimes they do fill up pretty quickly. Never fear, youthfulness isn't a requirement!
If I'm in Europe, I try to book in advance through Hostelworld - especially handy if you're arriving somewhere late and don't want the hassle of traipsing round looking for somewhere.
Hostels & Youth Hostels Worldwide - Online Bookings