There are several hundred hostels in Japan alone, more than in France or Spain. In nearly every part of the world, in cities and towns and in villages in the countryside, you can find youth hostels. The same membership card allows you access to all of them. Typical hostelers are under 30, but family and retiree groups also frequently use hostels. Children under 14 must be accompanied by an adult and some hostels will accommodate children under 5.
Youth hostels vary greatly.
Some, common in the US, are merely private homes with extra rooms. Others, more common in Europe, are public buildings which the local Youth Hostel Association has turned into hostels capable of housing dozens or even hundreds of people. Some of the nicer ones are even renovated historical structures such as castles. Except for those designed primarily for hikers, hostels can easily be reached by public transportation and many are just a short walk from a nearby train station. Wandering a little further away may yield a cheaper rate.