I found that after 21, the next Birthday (capital B) is 25, and then 30, probably, and then I think it goes by 10's. Growing up you not only have people more likely to emphasize your birthday, but you also have more milestones: When you're very young, every year is a lot, then you have the double digit milestone, then the teenage milestone, then 16, then 18, then 20 and 21....