Ymir is 75+ years old.

Titans showed up only 100 years ago.

That means Ymir has been alive since almost the beginning of the walls formation.

It is also shown that she knew the unknown language written on the can.

I feel like Ymir knows a lot of things that the Eren and the Survey Corps don't.

But the question is-

How did she die if she was inside the walls? 

During that time, if you read Attack On Titan: Before the Fall, then you know that they only used canons and medieval swords.

Was she apart of the Survey Corps at its early stages?

Out of what we know, in the Before the Fall, we've only seen adults fighting but no teenagers(except for Kuklo).

Before the Fall was set 70 years before the fall of Wall Maria which was also the time around Ymir's "death".

She also "died" when she was most likely around 15-17.

There are many unanswered questions that circle around Ymir.

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Kuklo is known as the "Titan's son" because he as born from a dead woman that was thrown up by a Titan.

I believe Kuklo got his strength from the Titan.

His mother's clothes and body were intact which means she wasn't severely injured or burned from the acid inside of the Titan's stomach.

That means the only way for her to die inside the Titan was from drowning on the fluids inside of its stomach.

Again like a said a few sentences ago, he got his strength from the Titan.

I think while in his mother's stomach, he absorbed the fluids coming inside of his mother which gave him amazing strength.

But before he could die inside his mother's stomach the Titan threw up and he was born.