Similar to "back in the day". Usage signifies that you have known said person for a long time; technically since you were little kids who were only as tall as the lower leg of an adult. Your height was "knee high".
Aaron: Hey man, that kid Sam is a realchiller. How do you know him?
Ben: He's been my boy for a while. I know that kid from knee-high
While our lifespan is the number of years we will live, our healthspan is the number of healthy years we will have and our sexspan is is the number of years we will continue to be healthy enough to continue having an active sex life.
A lighthearted reaction upon receiving disappointing news of some sort. Usually something that isn't a particularly a big deal, esp. when it primarily affects someone else - sort of a mild schadenfreude. Sometimes doubled or tripled, and possibly said in a sing-songy voice for added comedic effect to imitate a muted trumpet, which is how the meme originated.