This happened to me when I installed Age Matters mid-game. I was able to remove it by using Character Editor and manually removing the trait from everyone's health tab.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=1874644848

It sounds weird, worst case you could disable 'Fertility from life stage' and 'Fertility from old age' if you still see it is would not be from my mod, if it is 'Age Matters' for you aswell, like @oh no - said, I would ask over there, but with all the new 1.3 weirdness things will calm down a bit is my guess

Yeah, I'm having a similar issue. I have a starting pawn that is 13 years old, and keeps getting the "not reproductive" hediff, EVERY TIME I LOAD THE GAME. It's not that big of a deal, since I can just use Character Editor mod to remove it, but super tedious to do it EVERY time. But I can't find any option in this mod's options to set a minimum reproductive age. I'm assuming for now it's a 1.3 issue arising from 1.3 being new and still getting updates, but would be nice for this to eventually be fixed and/or a mod option added to choose minimum reproductive age. I know the hediff is from this mod, so am assuming it is being added by this mod until I can figure otherwise.

Edit: Actually, it is happening even without loading the game. Seems to be some check every so often, when it doesn't find hediff mod reapplies it...

Originally posted by I blame Pudge:

Yeah, I'm having a similar issue. ..

You can disable 'Fertility from life stage' under 'Fertility' it should no longer add it at that point (but also remove it for all), the issue itself might even be a vanilla issue, I expect it to calm down with time aswell.

(But it is very likely only a visible symptom of the lifestages beeing messed up in some way by something, is this a human or some AlienRace?)