There are milestones in life.
The day you notice the sign behind the cashier "You must have been born before..." has the year you graduated high school.
A few years later, when it's the year you were old enough to buy liquor.
The birth of your first child, when it suddenly hits you - it's no longer all about you.
The birth of your first grandchild, when it suddenly hits you - Oh, crap! I've got to baby-proof the house again!