Tumblr - The copy inside the age verification form, when signing up for Tumblr, changes depending on the value.
Google Images - When searching for some images which are somehow connected with recent events, it shows a little label with its age
Facebook - When entering an old password, it tells you when/where you changed it, and offers you to request a new one, or try again.
pinboard.in - When changing your password, the new password text box uses a low contrast font and forgoes the typical type-your-password-again pattern.
/via Lenny Sirivong