[WindyLakeReturn]

Is this age range the ones most commonly sending photos, or the one most commonly caught (as a photo sent by a 16 year old is more likely to pass as 18 and not be caught by any detection software or manual detection)?

At face value, the idea that this specific age group is sending more such photos than the older cohort is surprising enough I would reject it as bad data without further research into why it is happening. Selection bias seems a much more likely explanation.