Age 19 to 25 - Yay! Party with your friends all the time! Freedom!
Age 25 to 30 - People in your social circle start getting married/having babies. Hang out less. Party a ton less. Boo!!
Age 30 to 35: Divorces start happening and the newly divorced seriously want to party. People's kids start going off to kindergarten and they have more free time and want to party. Yay! Party with friends all the time! Freedom!
Age 35 to 40: Your friends who still partied with you between 25 and 30 now get married/start having kids. Hang out less. Party a ton less. Boo!
Age 40 - xxx: See age 30 to 35. On difference is that a fair percentage of the ones who had kids young will now have grandkids young, which will make you feel super old, but won't stop them from partying.
(In certain cultures subtract five years from each bracket.)