Nothing actually changes for you, but for other people - especially the latest college generation - the difference between you at 29 years old and 364 days and 30 years old is like 5 decades. If you tell one of these people you're 29, you're the cooler, sort of older dude in the crowd. Totally hip. If you tell them you're 30, you're that guy who probably lives in the suburbs with a minivan, two kids, and a seat on your school's PTA.