Um, what about the possibility that Jan 1 is the default and 92% of people are too lazy to switch their real birthdate.
I think it's obvious that Gabe is being pretty sarcastic.
Considering he created and runs an incredibly successful game development and distribution company who's target market is people under 25 years old it's pretty reasonable to asssume that he has half a clue about his target market.
comScore also released an announcement a while ago saying that gamers actually represent all markets segments, showing the potential of in-game advertising (which has been growing both in revenue and number of advertising in the past years).
So Gabe's target market should be a very broad one not only gamers "under 25 years old", specially since his main product is a platform (Steam).