Why Is Everyone In This SF Story Twenty-Four Years Old?

...or frequently even younger.  And usually male, of course, without any family ties. (where are the aunts and mothers?  The cousins and brothers and sisters?  The grandmothers?)

Athena Andreadis, over at Astrogator's Logs, writes an excellent post: "Where Are the Wise Crones in Science Fiction?"   

Bonus story in the post, also!