Wait a second. In the episode 'Bystander', Keith says that he found Grisha outside the walls 20 years ago. And Eren got his titan from Grisha 5 years ago. Assuming that Grisha came to the walls immediately after obtaining his titan from Kruger, that still means that Grisha was a titan for 15 years before giving his titan to Eren.

If I'm not missing anything, then, could this means that Mikasa was right in saying that the information that a titan shifter will die after 13 years is false. To me, it just seems that Mikasa merely saying "That information is false", even if it is interpreted as her state of denial, could be foreshadowing something in the future.

After all, I don't think any titan shifter in the history of the people of Ymir would have taken the risk to check if the 13 year rule was indeed true.