WARNING: The following article contains spoilers for the Attack on Titan manga.
The Attack on Titan fandom still can't come to a consensus about the Curse of Ymir, a rule which states that a Titan Shifter can live a mere 13 years after acquiring their Titan powers, and what this means for our protagonist, Eren Jaeger, who possesses more than one Shifter power.
This information was revealed in the second half of the anime's third season (and Chapter #88 of the manga) after the Scouts reclaimed Wall Maria and finally uncovered the secrets within the basement of Eren's childhood home. According to Eren's father, Grisha Jaeger, humans who eat intelligent Titans to gain their powers will have their lifespan shortened due to the strain of Titan powers on the body. The legend of the first Titan, Ymir Fritz, details how the Titans' founder lived for a mere 13 years, and thereafter, no Titan could surpass that limit. Grisha himself was said to be proof of these limitations, feeding himself to Eren after having been in possession of the Attack Titan for 13 years.
Fast-forwarding to the manga's more recent chapters, fans have received an update about Eren's own lifespan in Chapter #130 of the manga, wherein he claims to have only four years left. This tidbit has sparked a renewed interest in exactly how the Curse of Ymir works, and whether Eren is possibly an exception to the rule. So, let's dive into some possible theories about the supposed Curse.
Eren's true lifespan hinges on the answer to one question: Does possessing multiple Titan powers extend one's life beyond 13 years? There's evidence that supports the affirmative, but equally convincing dissenters have come out of the woodwork, as well.
As someone who gained the power of two different Titans prior to his death, Grisha Jaeger represents an interesting case. Before he inhabited the walls as we know them, Grisha accepted the Attack Titan from Eren Kreuger, an Eldian spy known to Grisha as The Owl. It was Kreuger who informed Grisha of Ymir's Curse and gave him the mission to steal the Founding Titan from the royal family of Paradis, the Reiss clan. By the time Wall Maria fell, Grisha had succeeded in retrieving the Founding Titan from Frieda Reiss before his 13 years were up.
However, could Grisha have lived for another 13 years after consuming the Founding Titan? By the 13th year, it's only natural that a Titan Shifter would be a bit worn out, but fans have pointed out that Grisha didn't appear to be weakened toward the end of his life compared to others in his same circumstances, such as the late King Uri Reiss. Grisha may have been able to overcome the Curse 0f Ymir, but he never took the chance to find out, leaving viewers with unanswered questions.
So, what does this mean for Eren? He could be exempt from the 13-year rule because he obtained both the Founding and Attack Titan from Grisha (and the War Hammer Titan later on), or his supposed immunity could be negated by the fact that he acquired both Titans simultaneously. The first Titan, Ymir, had the power of all nine Titans and died after 13 years, so Eren's multiple-Titan status may mean nothing.
Does eating multiple Titan Shifters simply give you more powers, or does it also boost your lifespan? Or would procuring more than one Titan wear out your body even more, further shortening your life? Although nobody but the mangaka, Isayama, knows the answer for sure, speculation within the fandom is unlikely to halt anytime soon without a concrete explanation.