It's awful! Basically within a day of starting Prozac, I was so angry and agitated and frustrated that I wanted to murder people. It happens when you lack the ability to metabolize some of the components in some antidepressants, either because you don't have the proper genetics or you haven't developed the mechanisms yet.
Homicidal akathisia is more common with teenagers because in some people, those mechanisms don't develop until adulthood. Which is awful, because teenagers also have underdeveloped impulse control. I was lucky that my mom listened to me and demanded a med change immediately. It can happen to anyone at any age, though.
This is also why it's important for us to report weird mood changes immediately when we're on new meds.
It happened for me on Prozac and Paxil, but it can happen on pretty much any SSRI and not happen on the next one. Someone else might be fine on Prozac but react like that on Lexapro.