Pretty simple. She is currently (physically) 17 years old, and returned to human form 5 years ago. Therefore, her pre-Titan and post-Titan age was mostly likely to have been 12 years old. Ymir did not appear out of thin air.

Is Ymir 2000 years old?

It’s said by Grisha’s father that the Great Titan war happened 80 years ago. So when Grisha was a kid, it’s been 1965 years since Ymir Fritz got her powers. In other words, it’s been about 33-35 years since Grisha was a kid, which means about 1998-2000 years since Ymir Fritz got those powers.

Is Ymir the oldest Titan?

Ymir’s gigantic Titan Ymir was the very first Titan and the only one to ever possess the complete power of the Titans; all who come after her merely possess fragments of her power. Ymir building the Wall Titans for Karl Fritz After Ymir’s death, her spirit lived on in a world where the Coordinate resides.

Well, I can’t answer your question directly but you also have to consider that Ymir has been wandering as a titan for about 60 years, thus making her a lot more “older” than Reiner or Berthold.