Its depend on different county rules and policy or Law. but mostly age is about 18 year or if your age is greater than 15 than you are allowed to go to youth hostel.but your age is less than 15 than you are not allowed to go to youth hostel.they will not allow you to stay in youth hostel.in mostly countries there is a law that if your age is more than 18 than you are allowed to go youth hostel.