In PA, you can become a donor via the dept. of motor vehicles. I don't know how it's handled in other states, but wanted to clarify my answer by stating such.
My husband and I are donors, but when my son turned 16, he was too uncomfortable with the fact that it's possible that he could die that he just couldn't check yes to being a donor. A year later, his sister turned 16 and she was quite ok with it, so she checked yes and is a donor (with our permission). I have little doubt when my son turns 20 and has to get a new license, he'll check yes, but I wasn't going to push that on him. I felt he needed to come to grips with it in his own time.
My husband and I are donors, but when my son turned 16, he was too uncomfortable with the fact that it's possible that he could die that he just couldn't check yes to being a donor. A year later, his sister turned 16 and she was quite ok with it, so she checked yes and is a donor (with our permission). I have little doubt when my son turns 20 and has to get a new license, he'll check yes, but I wasn't going to push that on him. I felt he needed to come to grips with it in his own time.