Final answer:
Studies show that the percentage of teens having sex has generally decreased in recent decades, likely due to improved sex education, awareness of STIs, and changing social norms.
Step-by-step explanation:
Regarding the question of whether the percentage of teens having sex has increased or decreased in recent decades, research indicates that the trend has generally decreased. Studies by organizations like the CDC have reported a decline in teen sexual activity over the past few decades. Factors contributing to this decline may include enhanced sex education, greater awareness of sexually transmitted infections (STIs), and changes in social attitudes towards sex.