I think that was "truer" a couple of years ago; I don't think they are practicing that a lot now. One company/agency that was known for faking their models date of birth was Pinkygal, with their known "Fuji army", and they took quite some flak for that back then. Nowadays I don't think you can get away with that a lot. Ema Nakajima's and Akua Otsuki's agency, for exemple, is Enter-pro, which seems reliable and/or trustworthy for the most part. I'm not saying it doesn't happen anymore, I'm saying that I don't think that it happens anymore left and right. A lot of the girls are already 18 - with published birthdays - when they do their first video, so they haven't got much of a case in lying about it anyway.