Answer:
I would say due to the lack of access to protection for sex. Also, women who d0uche.
Step-by-step explanation:
Women under 20 have less access to condoms. They are also less likely to use them. I would also say women under 20 commonly have more than one sex partner which also increases the probability of ending up with PID. Think about college towns. I am sure they have higher PID counts than say a general residential area with families.