I made her turn out to be Ali's great-great aunt and have her still around even after Ali comes of age. (Hey, I'm 25, the age I made leaf eaters come of age in a fanfic, and my great-great aunt is still alive! It could happen!)
She was an original resident of the Great Valley, which, in my fan fic, was originally inhabited by Sharpteeth, and they drove them out. (Think colonists and Indians.) Both sides were brutal to each other. Eventually, the Sharpteeth were drive out and came to this place that became known as the Valley of Sharpteeth. The Meanest Ever Sharptooth, sent years later by the Valley of Sharpteeth for revenge, was killed by Doc.
The Old One, when she came of age, left the Great Valley and resolved never to settle there long term again, as she had too many painful memories.
After 115 years (yeah, now she's the REALLY Old One at age 120) and she's back to the Great Valley, this time because some fiend named the Terror of the Mysterious Beyond (long story on him. Suffice it to say that he helped Sharptooth attack Littlefoot's mother and he's a total putz and S.O.B., not to mention pure EVIL.)
After her return, her great-great niece and Littlefoot fall in love, though it takes some time for Ali to like Littlefoot more than Rhett.
Meanwhile, Chomper and Ruby, now near adult age, well, Ruby is an adult at this point, though still single, are running from Red Claw, Screech, and Thud and a Fast Biter army. They might been axed, had not this helpful Sharptooth (the same species as Chomper) scratched Red Claw. They get away and Chomper, Andromeda (the helpful Sharptooth), and Ruby go to the Valley of Sharpteeth.
Chomper finds out the rivalry between the Valley of Sharpteeth and the Great Valley and says he thinks the residents of the Great Valley are nice, which infuriates some of the residents of the Valley of Sharpteeth.
Eventually, Andromeda takes Chomper leaf eater hunting, thinking he's never been before, as he has eaten leaf eaters in 15 cold times. (Ruby is a bit slow on that, luckily, or perhaps cannot do math as it wasn't invented yet and so realized that 20 - 15 = 5 and wonder about those 5 years that aren't in there.) Anyway, at one point, they end up gong after Littlefoot, Cera, Ducky, Petrie, Spike, and Ali. Chomper recognizes them all and gets Androemda to spare them.
Things go on, until finally, Andromeda being nicer toward leaf eaters, they bring her in an entrance to the Great Valley that the residents of the Valley of Sharpteeth know about but that the Great Valley residents don't.
Chomper, who is allowed in already due to his previously being there many times, is already there, waiting to hide her till morning, point out that she never attacked anyone, and that she thus is good and can be trusted to stay there. However, the villain spots Andromeda coming in at night and tips off the Old One, hoping to cause trouble.
Andromeda meets the Old One, and tells her she's from the Valley of Sharpteeth but that she means her no harm. (Androomeda's leaf eater is poor at this point so she talks worse than Petrie.)
However, the Old One, having lost friends to the Valley of Sharpteeth over 100 years earlier, and not willing to bury the hatchet, goes into "kill the savage" mode and Andromeda gets beat down.
Andromeda is about to end up defending herself, but Chomper, who had been on the verge of telling Littlefoot & co a secret he learned about the villain, one that could end the villain's evil reign as it would expose him for the creep he is, when he hears the fight.
He tries to get the Old One to back off, but she is too determined and is about to kill Andromeda. Chomper leaps at the Old One, intending to push her away. However, he misjudges his aim and accidentally impales her in the stomach with his sharp claws. This gets blood on his claws and he has to fight off his renewed carnivorous nature (let's face it, eating hoppers, buzzing stingers, and swarm leaf gobblers can't be very filling for a near adult Sharptooth). He comes really close to sinking his teeth into the Old One, but backs off at the last moment.
However, the Gang of Five, plus Ruby and Ali, spot him. Only Littlefoot saw the fight in more detail, as he got there sooner. The others believe Chomper hunted the Old One.
Chomper and Andromeda flee and are met by Littlefoot near the secret exit to the Great Valley.
There, Littlefoot and Chomper talk.
Chomper learns, after Littlefoot mentions where he found his egg, that it was actually Littlefoot, not Ozzy and Strut, that took his egg to the Great Valley.
Littlefoot, in turn, learns of the five year time that Chomper ate leaf eaters and that Chomper had, in fact, been hunting them in V and that was what that chase was all about, but he had realized that they were his friends and so backed off. Littlefoot asks if they had been Ali instead, if he would have eaten her. Chomper nods. Littlefoot goes into a rage and says "Fine then, go be a Sharptooth!" and Chomper, hearing the other dinosaurs coming, flees.
Littlefoot goes back to the Old One. She only manages two words "Littlefoot! Ali" before succumbing to her wounds. It looks like Littlefoot and Chomper's friendship is over, not to mention, the incident causes a, thankfully temporary, rift in Littlefoot and Ali's relationship too, but fortunately, it is later restored, as is theirs with Chomper, Chomper pardoned and living in the Great Valley as a permanenet resident with Andromeda, and the villain is killed.
Still, that was my take on what happens to the Old One in the end.
