The question of age plagues anime characters in all aspects and Pokemon is a curious case with Ash being eternally ten years old but how old are his companions, more specifically Misty.

In most cases, Misty is simply the same age as Ash and most information sites agree with that theory.
But the Electric Tale of Pikachu makes the tomboy mermaid 13 years old which is much more accurate.
But I'm willing to bet that Misty is actually 12 years old and I have a few things to back that up.
First is her body type, I'd hardly call it childish but wouldn't call it teen either.
Second is her maturity, yes you can have a mature ten year old but Misty does drop subtle hints that she knows about certain adult things, think back to the episode Holy Matrimony and Jessebelle's dungeon with the so called "weird gym equipment", blink and you'll miss it, Misty blushes witnessing what Jessebelle is trying to do to James.
It's obvious that Misty knows what's going on even if the dub hides it, she has three older sisters with Daisy being a legal adult, one of them has done the dirty.
The third one is how close she has been with Tracy who is an obvious teenager with an implied romantic link.
Finally Misty's video game counterpart would suggest that my age estimate is pretty spot on as well especially in Masters Ex where she is noticeably older than player protagonist Leaf.
No matter how much you say it, Misty looks too old for ten years old, I can believe May, Serena and Dawn are ten, but not Misty.