Slashdot videos: Now with more Slashdot!
It's very much used for tax-papers, in particular, and from a technological point-of-view, I don't think it's unreasonable to build an identification infrastructure for social networks around this.
The obvious privacy issue could be addressed if the verification entity was independent and trustful, much like we see Certification Authorities (CA) for SSL. These authorities migt then be trusted to only hand out the age of an individual signing up for the account. This would, in effect, act like an age barrier.