Cast iron corrodes from the inside as decades of waste and moisture pass through it. Scale builds up along the interior wall, gradually narrowing the pipe’s usable diameter across the whole run rather than at one blocked point, while the same corrosion eventually pits all the way through, opening leaks at low spots and joints. Because the narrowing happens everywhere at once, a home on cast iron typically shows several sluggish drains together, not one obvious clog, which is itself a useful clue before a camera ever goes in.
Age alone does not confirm anything, since a pipe’s actual condition can vary widely even within the same run. That is why we run a camera through the line before recommending a repair path: it shows the scale, the pitting, and whether the pipe has collapsed anywhere, and that picture is what determines whether a targeted patch, a trenchless liner, or a full replacement is the right call, not a guess based on how old the house is.