I recommend interesting book to everyone who is interested in what kind of people make the products you buy: Leslie T. Chang "Factory Girls".
It among other things makes it obvious that these youngsters are pretending to be of legal working age of 17. They use ID cards of their cousins and what not to gain access earlier.
What "should" be the responsibility of the factory would be similar to a bouncer in nightclub: you know people who are underage are trying to gain access, so you should be vigilant about this issue and do check the IDs more carefully.
BUT we do know that like many nightclubs, so do these factories, welcome these "patrons" in any case, maybe they need their input in one way or another etc, at the end of the day even though they suspect that there are underage people inside their facilities. The Chinese factory middle management, line management etc. most times don't care. They got their quotas and so be it.
As said, a recommended human story book on the positives and negatives of normal life in South China factory city, read Leslie T. Chang's book.