There comes a time in a man's life

When you're out walking around your neighborhood, and you see a bunch of signs in a yard celebrating someone's 40th birthday.  

At first you smile, then it dawns on you when you see the 1985 sign that 1985 was when you graduated high school.  That is, the person who is now 40 was born the same year you graduated.  Then you continue walking, but more keenly aware of the aches and pains with each subsequent step.