Due to to my lazy nature, i input whatever date is the easiest. not like it actally matters anyway. on 'agegate' i remember i ended up having 1/1/1900 as it was the easiest one to get with a couple of clicks, if its a scroll down list i just go somewhere beyond the 80s.
if a website/forum requires it, and there is a good chance they will request it for recovering lost passwords the like(oh, how i abhorr that practice) i use the real date if i ever expect to have to recover it. odds of remembering a fake on tend to be rather low.