I'd say a generation is the time for a newborn to grow up and have their own first kid born. Typically that's been under 20 years. May be changing now, getting later, but I usse ~20 years as rule of thumb for one generation.
We have lots of examples like that, for instance lots of people complain when "a couple" refers to any number other than two, and yet common usage is for it to be pretty synonymous with the more obviously vague term "a few".
Another reason to be strict with output and flexible with input.