In Chapter 71, Shadis says "I met Grisha... twenty years ago now?", meaning the year would be 830, but according to the pages on this wiki this happened 18 years ago and not 20. Meaning Zeke is 25 and not 27... was this correction made at some point in the manga or did someone think it was 18 years because of Ymir's Curse?
Shadis' tone suggests that he's uncertain of his own claim that it happened twenty years ago. Because of the 13-year curse, Grisha couldn't have been within the Walls for longer than that. Since he went with his choice to attack the royal family in the year 845, assuming he'd reached his final year, this means he would have arrived in the Walls in 832. 18 years is pretty close to 20, so Keith was just uncertain of the timespan. It could be that he hadn't seen Grisha for several years after finding him, which would've thrown off his memory of how long he'd known him.
That's definitely one of the things I considered... might have to look into the raw and the different translations though. Still, I think Ymir's Curse has some unexplained aspects... though considering Grisha wanted to show Eren the basement if he made it back, it's possible it was his last year... hmm.
This just happened in the anime and seeing the 20 years confused me, knowing about the Curse.
Option 7: The Attack Titan is unaffected by the Curse of Ymir. It would go with the theme of them breaking free/advancing. It wouldn't necessarily explain why previous Attack Titan possessors might have died at 13 years, but as far as I know we don't have any concrete examples for it happening to Attack Titan holders, let alone many others. Eren Kruger may have just thought that it would affect him, either because it's a rumor or because it actually does affect the other Shifter Titans, so he gave it up so that he would know it went to someone who could do something, rather than a baby (if he died).
I would also tend to agree that using the power more would make you age faster, but I don't know if that applies to the Founding Titan, which seems to not use it that much but that consistently seems to get very aged after the 13 years. Or maybe they really do use memory changing a lot more than we realize.