1, 13, 16, 18, 21, 40, and 50.
1, because you've been alive a year.
13 because you're a teenager
16, you can drive
18 you can vote and claim yourself independant, buy cigarettes, and buy p0rn
21 you can drink
40 you're 'over the hill'
and 50 is well, fifty.
For me 16, & 21 were monumental.
25 years old saw a decrease in car insurance rates, but that only applies to single males. Males that get married prior to turning 25 see the decrease at that time, and females are already at a lower rate.
Other than that from what I have seen once your 21 there really isn't even a point in having a birthday.
God knows I haven't seen a birthday party since 21.