Both my children are millenials. Among other things I'm a data security professional, and I taught both of them as much as I know about privacy concerns, personally-identifiable information (PII), and online reputation (although I didn't use that term, exactly). My wife is an attorney, and has also provided information about online reputation and privacy concerns.

My 18-year-old daughter doesn't seem to get it; she divulges personal information all the time. My 17-year-old son, on the other hand, is quite careful about what he posts. In this small sample size, my daughter is not an "IT person", while my son is (he's taught me a few things), so that may have something to do with it.

I've not done any surveys of this behavior; however, in my small world (knowing a few hundred people) most people I know do not divulge private and/or PII online, regardless of age... but my experience shows something interesting. Teenagers are less concerned about such matters than adults, but they are more aware of doing so. Adults are more likely to divulge things because they don't realize they're divulging things, so the two may even out in the big picture.