Prosopagnosia in Age Determination?

In an article on Slate.com about a new law in Tennessee that requires anyone who wants to buy alcohol to be carded, regardless of age, because of the lack of ability of clerks to accurately determine age. There is mention of prosopagnosia in the final paragraph.

Can a bartender tell if you're underage? - By Torie Bosch - Slate Magazine: "The new law might be helpful for any Tennessean store clerks suffering from a disorder called prosopagnosia, or 'face blindness.' Prosopagnosia, which is sometimes associated with a stroke, autism, brain damage, or other neurological disorder, can limit an individual's ability to estimate age at all."

The frustrating thing is that the term prosopagnosia is introduced, but not explained. It is only linked to the lack of ability to estimate age, which is more a possible symptom of not being able to recognize/remember faces in general, and certainly not attributable to prosopagnosics in general. It also does not mention it can be developmental, as well as acquired through brain trauma.