Chronologically, Zoe is between 1547 and 1396 years old, although she still looks like a teenager. While her biological age can be a bit hard to pinpoint, looking somewhere between 15 and 18, Zoe's chronological age is an easier guess, Her lore actually gives us a pretty reliable timeframe for her age.
"The Aspect hadn’t seen such joyful irreverence in the face of peril since its last host, who heralded the end of the Great Darkin War."
The previous Aspect of Twilight was at least active at the end of the Great Darkin War, which happened in the year 550 BN (Before Noxus).
"Fortunately, she could traverse the streams of time with only a thought. Among the events she witnessed were the rise and fall of “the big armored meanie,” Mordekaiser,[...]"
Zoe became the Aspect of Twilight before the rise of Mordekaiser, which happened in the year 400 BN.
This means Zoe's birth year must be somewhere between 550 BN and 400 BN. Given that it seems a long time has passed since the previous Herald of Twilight had been chosen, the year is probably closer to 450-400 BN than anything else. With the current year in League of Legends lore being the year 997 AN (After Noxus), this places Zoe's age sometime between 1547 and 1396 years old. Zoe remains with the body of a teenager since only a year had passed for her since she ascended. However, she also shows in-depth knowledge about physics, celestials and the universe in general. It's not an unlikely theory that, unlike other Aspects, Zoe was so tightly knit with her Aspect that she was able to fuse completely with the Aspect of Twilight, rather than be possessed by it or coexist with it. In that case, Zoe's mental age remains even more nebulous. If she really fused with her Aspect, then it may be more fair to say she's completely transcended the concept of age or time.
As the current Targonian Aspect of Twilight, Zoe is the cosmic embodiment of mischief, imagination, and change. As befit her aspect's nature, Zoe did not come to the attention of the celestial realm in a traditional way. She didn't sacrifice herself as Pantheon did, didn't overcome the trial of climbing Mount Targon as Taric did, nor did she dedicate her entire life to an ideal like Leona. Instead, Zoe was a normal and average mortal girl, chosen seemingly at random from among the Rakkor.
Her teachers reported Zoe to be an imaginative child, but willful, lazy, mischievous, and easily distracted. One day, as she skipped away from her studies of the holy texts to pursue something “less boring,” she was noticed by the Aspect of Twilight, who chose her as the new Herald of Twilight after being impressed with her joyful irreverence, choosing instead to double down on mischief rather than save her skin. Instead of laying down a test for the girl, the Aspect brought her to the top of Mount Targon directly, for her ascension.
This transcendence was unique—in fact, it was unheard of in all the myths and legends of Targon. Yet Zoe did not trouble herself with why the rules that govern Aspects had been changed just for her. Instead, she journeyed to dimensions of reality at the very edge of mortal comprehension, playing with powers seen by few before or since.
While for Zoe barely a year had passed, she returned home after what had apparently been many centuries in Runeterra. She traversed the streams of time with only a thought, witnessing the rise and fall of Mordekaiser; the destruction of the Blessed Isles, the Rune Wars, and finally, the founding of Noxus.
Zoe eventually found herself growing distant from other Aspects, instead traveling to the cosmos and meeting Aurelion Sol, who detested the Aspects of Mount Targon for taking away his power. Aurelion Sol and Zoe remain in a somewhat calm relationship, the dragon at the very least having stopped threatening to destroy her when he takes his vengeance.
Exploring Zoe’s lore and abilities can be even more exciting when paired with gameplay. For a fresh perspective, consider starting with a level 30+ unranked LoL smurf account, perfect for testing her cosmic tricks while climbing the ranked ladder.
Regardless of when she was born or what she actually is, one thing remains evident: apparently, Zoe does not quite age. Her description of the things she had missed in Runeterra only covers the stretch of time from the rise of Mordekaiser to the founding of Noxus.
While this is a pretty big time skip, there are also 997 years between the founding of Noxus and the current year in the canon. This does mean that while Zoe is physically somewhere in her teens, she has lived through events like the first war between the war between Demacia and Noxus and the Noxian invasion of Ionia. Moreover, a quote from Bolbok in the first season of Arcane mentioned that Piltover was "founded to escape the warmongering of mages". Given that Heimerdinger says that only he carries the knowledge of what happened in the Rune Wars during the council, it's unlikely that this is what he was referring to. It’s somewhat more likely that Piltover and Zaun were founded to escape the wars between Demacia and Noxus (given that the higher class of Noxus does cozily fit the description of “warmongering mages”), meaning that Zoe was also alive to see the founding of those cities as well.
It's unknown if ascending into an Aspect changes one from a mortal to an immortal, or if this would prevent aging, but it's also likely that it could be a result of Zoe fully merging with her Aspect as one immortal being, or that the Aspect outright granted Zoe immortality as a boon to their herald.
