I never understood either how ~20 years constitutes a generation. Fads change much faster than that, so a guy 18 years older than the next guy is much different.
What a stupid definition.
Nominally, our current society defines an "adult" at age 18. Coincident with that age, marriage (or not) and procreation create the next generation. My parents didn't start having kids until age 28. My grandparents waited until age 43. WWI delayed the opportunity. My great grandfather married at age 25...in 1865...after serving in the Union Army. He raised 19 kids from 1865 through 1887. He actually lived until 1921.
A "generation" is a mix of biology and societal expectations.