As far as I know, the Survey Corps (and other relevant Elderians) learned of the Curse of Ymir (that a titan shifter only has 13 years to live and if not eaten, will die to have his titan power born again with a random Elderian) from the memories of Grisha Jäger, who in turn learned it from the Owl. The Marleyian army seems to be aware of the course as well. Owl probably got his information from the army or the Taybar family.
Now I wonder, is it guaranteed that this information is correct? From a narrative POV I don't believe the author will just wave his hands and say "The curse never really existed, sorry!", but from an POV from within the world, it would make sense that this is misinformation, deliberately given by King Fritz to protect Elderians outside Paradise (so they are not deliberately killed off as long as they can be used).
I want to know if there is hard proof that the curse is real? This can be witnessing it in the main series or spinoffs (I didn't really follow AoT - After the Fall so much) or in interviews with the creators.
Something ominous, deliberately question-dodging like "Well, the characters got told so." in the interviews would be interesting too. After all, the characters got also told that titans are mankind's enemy, that you die if eaten by a titan, that Paradise Elderians' are devils etc. Information given in AoT through characters cannot be trusted, as we know after all these years :D