Rarely - anymore I just scroll down the Years column until I hit something old enough and click submit. I'm 27, so it's not like I have to lie to get past age gates, I'm just in a hurry and it takes marginally longer to select my actual birthday on a screen I'm only interacting with at all because I want to get past it.
The entire concept strikes me as silly really, since anyone can put in any date they like - if a page has content inappropriate for minors, it should just warn you of that and have a button to click that acknowledges you're aware of it and takes you to the actual content, and one that lets you navigate away. Making us pick things from drop down boxes is more annoying and presents no more obstacle to minors than a Yes/No option would, so if there is nothing to link responses with individual users, why bother with them at all?
On actual communities though like this one? No reason I wouldn't use my real date of birth really. My real identity is practically an internet ghost in the sense that while you can find me online, the only way you'd know you'd found me is if you already knew all the details about me you would be using the internet to find in the first place, so a real birth date coupled with a pseudonym and vague factoids doesn't get you very far. Hell, using my real name in Google image search doesn't even turn up a photograph (of me, it turns up all sorts of complete strangers though) - there are so many false positives that it isn't even funny.