It if makes me type, I type in my actual birthday as I find it quicker to just throw that in then type a random year. If it's a drop down menu, I'm always born January 1st, whatever year in the 1900's I scroll to and select because it's faster roll to throw the scroll wheel on my mouse a time or two and pick one than sit there looking for my proper year.
The sad thing is that a lot of these things I run into anymore are on services that already made me log in to use and have likely already made a record of my birthday when I signed up, or at the very least could take a moment to have you enter your real birthday and actually remember it so you don't have to re-enter it all the time. Like Steam, I was looking in the sale catalog the other day and they had a Doom pack on sale, and I wanted to look at that so I clicked, and then it's asking me for my age. Okay Steam, I have an account, and I'm signed in as soon as I launch your software. So how about you just let me input that I'm currently 24 and we skip all these silly age checks every time I want to look at an M rated game's page?
Strangely enough, all the porn sites I've seen don't bother, they just say "by entering this site you agree that you are of legal age to view this material in your country" and don't even bother asking.