Age verification is commonly performed by identifying the subject
(name, address, card number, etc), and looking for information which
validates or corroberates that the subject exists and is likely to be
of the age claimed.

I want to find a web service verifying the claimed age of online
visitors without access to any external source of information.
Alternatively, I would like to access research papers or reports
describing such an attempt. I assume that this can only be done by
asking the subject additional questions,and perhaps utilising some
sort of online lie-detection technique.

I require at least one web address or some other reference in your answer please.