You guys are making sweeping generalisations about things that are intimately tied to local culture and traditions and therefore vary greatly depending on who you ask and where. Half the world thinks that if you're not married by your early twenties you're reaching your sell by date, the other half lived at home until their mid thirties for any number of reasons. People now have hobbies and passtimes like videogaming that until 20 years ago was considered to be a children activity, and they keep playing into their retirement.
Either way you look at it, none of it is relevant to DS, a game set in a dystopian/future setting, after an apocalyptic event that has given birth to, amongst others the decayed, who have an indefinite lifespan, not to mention mutants.. Who knows how old mutants are and if their bloodlines have ever entered the bloodstream of "normal humans"?
(Well.. I know, but y'all don't )
Also, the mortality rate is such that dying of old age or typically age related health conditions is by no means an easy feat..
Finally, Zeta is the product, as a society, of a survival of the fittest culture, which has gone on for several centuries and counting.
So any sort of real world parallel doesn't apply.