A lawn mower should last between 5 years to 10 years and even sometimes as long as 20 years and sometimes more if taken care of properly.
I have a 20 year old riding lawn mower and a 20 year old push lawn mower that starts the first time every time and I've had them both since new.
I've changed the oil when needed and kept gasoline out of the carburetor and tank when in storage.
Some people buy a new push mower and then use it for 1 year and then leave the extra gasoline in the tank over the winter which then causes problems with the push mower starting.
Usually cleaning the old gasoline out of the tank and cleaning the carburetor up can get it running again.
However if you do get 10 years out of a push lawn mower then that's pretty good too depending on how much grass you've mowed with it over those years.