Final answer:
The most common age group for Cervical Intraepithelial Neoplasia (CIN) is among women aged 25-34 years, due to the typical timeline of HPV infection leading to CIN and screening guidelines starting at age 21.
Step-by-step explanation:
The most common age group for Cervical Intraepithelial Neoplasia (CIN) is typically among women aged 25-34 years. Although HPV infections are widespread and can occur in individuals as young as teens, the progression to CIN usually takes some time, meaning that HPV's initial infection later leads to CIN during this age range. Most clinical guidelines recommend starting routine Pap smears at age 21, which is essential for the early detection of CIN and cervical cancer. The focus on screening starting at age 21 and the nature of CIN development supports the prevalence of CIN in the 25-34 age bracket.