What is a sewer scope?
A sewer scan is done by accessing the main sewer cleanout and a fiber optic camera is fed through the sewer line that runs from the house to to the city sewer at the street. This sewer line belongs to the property owner, and any problems are the financial responsibility of the owner. If there are problems with the sewer line that routine sewer cleaning won't solve (i.e. tree roots slowing things down), the repairs will be expensive.
Unfortunately, no home, no matter its age, is immune to sewer problems, but as you can imagine, the potential for problems increases with the age of the home. There is no clear dividing line between homes that should and shouldn't have a scan performed. If your buying an older home, it's probably smart to have a sewer scan done, your decision will ultimately come down to the buyer's tolerance for risk.