Is there hard proof for the Curse of Ymir?
I noticed that Grisha Jaeger did not suffer the rapid aging that other titan shifters experience in their later years. There is one major variable however, Grisha didn't use his powers nearly as much as the others, or suffer major injuries very frequently. 
  Aging is caused by the inevitable genetic screw-ups that occur during cell division. The Matter and Energy may com from Paths(whatever they may be). But instructions for building the titan titan bodies and growing back limbs? That comes from the shifter.

Grisha Only used his titan form twice(Getting to the walls and supper with the Reiss family), and he doesn't get major injuries like a soldier(You know, like Eren Kruger) might experience. It would also explain why Grisha slacked off on his mission for 12 years until he heard about some big-ass titan kicking a hole in the wall.