What are traditional milestone birthdays?

Answer 1

Birthdays : 1 (first birthday), 10 (double figures), 13 (first teenage year, bar/bat mitzvah for Jewish teens), 18 (some laws incluing smoking *in certain states, drinking *in some countries, driving), 21 (driving, smoking, drinking depending on government and country), 25 (quarter century), 50 (half a century), 75 (three quarters of a century), 100 (a century, triple figures).