1st is always celebrated. 2, 3, 4, 5, 6, 7, 8, 9
Teenage years. 13, 14, 15, Sweet 16, 17, Debut for girls and Legal age at 18 for both boys and girls.
No more teens. 20, Debut for boys at 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31
Out of the Calendar. 32, 33, 34, 35, 36, 37, 38, 39
Because life starts at 40. ;)
And I'm sure it's just around the corner.