How does steam know my birthday? I've used Jan 1, 1900 for the last 20ish years. But today I went to a game and it was age locked, with the default set to Jan 1, 1900. Which is expected behavior, but when I clicked "View Page" I got an error saying I could not enter. I tried a bunch of wrong, but realistic, birthdays and it failed every time. When I picked my correct birthday it let me through, so my question is how did they get this? Where is this stored, couldn't find anything in account information. And I have never given my birthday to steam ever. I always lie to any site/game that has asked. Not that it matters since I'm considerably older than the minimum requirement now.

Did they change something recently?