You can find lots of different numbers on the lifespan of a cast iron drainage pipe, but I can say that at 80 to 100 years of age, the pipe needs to be replaced if a couple things add up. This type of pipe is commonly rusted. But “very rusted” is something more substantial. When the pipe is pitted or kind of bubbling from tiny fissures is a warning sign. Sometimes I see long cracks in the pipe and it’s leaking. That was the case with the pipe that I pictured. This one was under the kitchen sink. The main cast iron pipe under the toilet was also leaking so that’s another bad sign. The video plumbing scope showed that rust had fallen down from the vent stacks and the camera couldn’t even get through the line to see what was wrong farther down. All these add up to one thing that I shall report on paper; all the cast iron drainage system is at the end of its life cycle and it all needs to be replaced. I told the buyer that he probably doesn’t need another scope after the line is blown out, you just have to replace all the pipe and it will be expensive.