sgerene, hostels (called "hostales" in Spainsh as you already know) in Spain aren't really known for being places to meet young people although it is possible. I was talking to the owners of Hostal López
a few weeks ago and they told me the average age was in the 40s. I was surprised to hear this, but apparently people of all ages still want value for their money, preferring to spend more on things to do/see than where they sleep. It's logical.
To meet young people, you might want to instead consider a youth-hostel. I don't really recommend youth-hostels (called "albergue juveniles") because while you'll meet more young people, it's not always comfortable sleeping in a room with several people whom you don't know and there's always the issue of security/safety of your things while you're sleeping or out and about seeing the sites.
You might do better to stay in a hostel (called "hostal") and then go places to meet young people. Good luck and have a wonderful traveling sola throughout Spain!
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