It's written clear in her lore.
"Her Lunari teachers reported Zoe to be an imaginative **child**, but willful, lazy, easily distracted, and mischievous."
"It observed as the **young girl** playfully mocked the angry cries of the Lunari priests chasing her"
"Returning home after millennia, **Zoe has aged barely a year**. Though Runeterra has changed little from her perspective, she arrives full of teenage curiosity for humans and her fellow Aspects."
I've just seen a lot of posts like "actually Zoe is "legal" " and it always bugged me like "how she looks like shes 12." and reading her story finally cemented that oh no she is a child people who use the term "loli" are just being creeps again.
Lots of adult women in League look younger than they are. making her stand out less as an actual young person but still. Her eyes are half the size of her head and she skips and plays jumprope when she runs, blows raspberries , has silly kid crushes, its just. She's a kid being a kid.
Ezreals "cradle robber " quote certainly doesnt help but Ezreals not the smartest is he.