Ymir is officially documented by multiple sources (character bios on anime's website, character bio in the official guidebook) as being physically 17. She has now revealed that she has an extra 60 years tacked onto this, in which she did not age.
Ymir spent 60 years as an ordinary Titan, and did not begin aging again until she regained her human form. This suggests that while Ymir was physically and mentally 17, her chronological age was most likely between 70 and 80 years of age.
Ymir lived 60 years as a “mindless titan”, not a titan shifter. She was turned into a mindless titan after everyone knew she was lying that the blood of the king runs through her. After several years, she encountered 4 children, ready to destroy and attack the walls, Annie, Bertholdt, Reiner and Marcel.
She transforms into a Titan at 10, spends 8 years or so dominating Eldia's neighboring countries, bears King Fritz's children at 18, then dies at 23 when her own children are around the age of 5.
Due to the Curse of Ymir, we know Ymir Fritz died exactly 13 years after becoming the Titan progenitor, and that makes her roughly 23-years-old when she dies protecting the King. Dying as a 23-year-old fits Ymir's chronology following her capture by the Eldians.
To "reward" Ymir for her service, Fritz took her as his concubine and together, they had three children: Maria, Rose, and Sheena. It was speculated that during this time, Ymir grew to genuinely love and care for Fritz despite his behavior and attitude towards her.
Ymir Fritz obtained the power of a Titan as a young girl. Being the first person to become a Titan shifter of The Founding Titan. Ymir Fritz is said to be the founder of the Eldian empire and “the people of Ymir” which is the only race of people who can be turned into Titans.
For 2,000 years, the Founder Ymir has been waiting--a slave to those with royal blood. However, Eren's yearning for freedom gives her a choice that could shake things up.
Because it is impossible for anyone to surpass the Founder, each person who gained the power of the Titans was fated with the "Curse of Ymir" (ユミルの呪い Yumiru no Noroi?), which limited their remaining lifespan to only 13 years after first acquiring it.
Ymir needed them because she was only a child who was the victim of terrible abuse. She needed Eren to help her free up all her pent up anger at the world because of what she experienced. She needed someone that understood her and could relate to what she went through.
Ymir was named after Ymir Fritz, the ancient Founder and very first Titan in history, by a scammer to present her as a member of the Fritz royal family to an Eldian cult in an impoverished ghetto in Marley.
according to chapter 122, ymir was a slave and when she died, she waited 2000 years for eren to free her from being a slave? apparently she "pulled some strings" in the world so it could all lead up to eren meeting her in PATHS, just so eren could tell her that she's free.
After being brought back to Marley, Ymir willingly allowed herself to be killed and was eaten by Marcel Galliard's brother, Porco Galliard, transferring the power of the Jaw Titan to him.
That being said, as the founder of all Titans, Ymir has the ability to force her will onto her subject. And since Ymir can only use the Titan's power for 13 years, then every Eldian who received her Titan power would also only have 13 years to live their life. And that is the curse of Ymir.
Human form. Ymir was a tall, slender young woman with short, shaggy black hair parted down the middle. She had deeply intimidating gray eyes and freckles on her cheeks.
Ymir is weaker simply because she did not know how to use the full extent of her titan power and had no training to further develop her powers.
Before the operation to retake Wall Maria Keith Shadis tells Eren that he first met Grisha about 20 years ago. At this point the attack on Wall Maria which coincided with Grisha's death happened 5 years ago. So Grisha had in fact been inside the Walls for 20-5 = 15 years!
4 Perk: Pure Titans Practically Live Forever
In effect, Pure Titans are immortal. Combined with their physical strength and imperviousness, Pure Titans can withstand the ravages of time as well. Cases in point, Ymir and Grisha's Eldian restorationists.
Titan is not a pleasant place for life. It is far too cold for liquid water to exist, and all known forms of life need liquid water. Titan's surface is -180°C.
Who impregnated Ymir Fritz? To "reward" Ymir for her service, Fritz took her as his concubine and together, they had three children: Maria, Rose, and Sheena.
For 2000 years, Founder Ymir waited for someone until Mikasa appeared. Why was she chosen? Mikasa & Ymir both faced childhood trauma as their parents were killed, homes taken away & tried to be enslaved. However, their lives turned out differently as they met different people.
The Shining Centipede, or Centipede-kun as it's affectionately known to fans, is the insectoid entity that grants Ymir Fritz the Power of the Titans.
At the Animagic 2014 series panel, producer George Wada confirmed that Ymir and Historia are "indeed a couple".
Fritz used Ymir to build bridges and roads and cultivate the wastelands; he also used her power as a weapon of mass destruction against the Marleyans. He further made Ymir his concubine and she bore him three daughters named Maria, Rose, and Sheena.
Ymir was evil, and all the generations of evil frost giants in Norse mythology descended from him. A great cow named Audhumia (Nourisher) was also formed out of the ice, and Ymir was sustained by the four rivers of milk that flowed from her udders.