Another thing to remember: most teens in the 70s and 80s were "older" at 13, 14, 15, etc. than similar aged teens in the 90s, in the 00s, the 10s, and now.
I wouldn't assume Sherri had no income. In addition to the theory mentioned about her possibly staying with other young people--which I think seems plausible--she might have also worked. She could have had a fake ID, she could have done casual labor of some sort. Cleaning, waiting tables, fast food, that kind of thing.
Such a vibrant young girl. Her face looks so full of life and fun. I am so sorry for her surviving family and hope they learn more about what happened.