system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If incest is shunned due to the risk of birth defects, why isn't elderly procreation, which has the same problems?

Seems like a bit of a double standard. The only thing I can think of is that incest limits genetic variation, which is apparently problematic, but procreation of the "golden aged" (40+) demonstrates a massive rise in birth defects as well, further increasing with age, so why isn't there a similar stigma? If you told someone you messed around with your sister/brother/cousin, even without intent to create life, jaws would drop, but I've never heard of negative reactions following a 40+ couple having a kid. If anything, I would expect that to be rationalized as "good for the couple for taking the time to live it up first."
gpt
Nature kind of solves that problem.  Not many women popping out kids at 50+.  The window from 40 -50 is an overlap, but as age and potential defect rates increase in that window, the number of candidates also decreases. 

However the incest stigma is more than just a medically driven custom, it also places a social control on those of the younger end of the reproductive spectrum where impulsivity, and access to only relatives, would be more pertinent.