You are getting more experience by volunteering and taking classes, which will definitely help in the interim while you search. You don't have to wait until you are 20. The rejections are not personal against you; it has to do with a specific skill set they are looking for, and that with the things you are doing over time you will gain if you don't already have them.

This can even be a good question for interviews, 'What skills make your employees successful?" / "What makes for a successful employee?" Then you can relate the skills you currently have that address being a successful employee or how quick a learner you are either in getting the skills currently through classes or for on the job training; that you are always willing and eager to learn. This is what helped me to get my first position that I really had minimal skills for, and then they were willing to train me because of an active interest and desire to learn. The other advantage to asking is that if you are not the right candidate for the position, you will learn what the skill set is needed to curtail your education and to get the skills you need if this is the type of position you want to go into in the future.

Though a rejection feels personal, it has nothing to do with you as a person; it is just being not the most qualified candidate at the moment and jobs are so limited right now with the economy. You'll get there doing what you are doing with volunteering and classes in the meantime. Hang in there!