Yep, this happened to me a few weeks ago (even commented about it on a HN thread). I've had this youtube account since about a year after youtube came out and the google account its attached to since about 2005. To not be under 18, I'd have had to be be rather young when I created those accounts.

With all the privacy-invading data google and youtube collect on me, surely they know that I'm old enough to watch whatever I like on youtube. But there's no way I am using either of the available methods to verify my age. I'd rather just not watch stuff on youtube.