Twitter would like to wish you a happy birthday. Twitter would also like to make it a lot easier for the people who buy ads to target you. Lucky for them these things are not mutually exclusive. Rather, they are inexorably linked.
While your Twitter profile has always been refreshing in its sparseness, a new detail has now been added. You can add your birth date to your Twitter profile. You have control over who sees it if you choose to put it in at all. You can also share your birth year separately from month and day, if you don't mind sharing the date without specifying how old you are. And once you decide to opt in Twitter can put fun balloons on your profile page to help you celebrate. Isn't that nice?
Of course your birth date is also a major piece of demographic information that advertisers would love to have access to. By knowing how old you are they know which advertising they want to target to you. Even if you decide to hide your birthday from everybody on Twitter, it will be shared with advertisers.
Still, if we can't avoid seeing the advertising then maybe it's best to focus the ads to things we might actually want to see. It seems like if you said it just that way a lot of people would still give Twitter their birthday info. The balloons makes it seem like they're trying to put one over on us. Just be honest Twitter, we know how the game is played.