This question always comes back to me at some point when I reflect on the show. How old is Jade? There are a few clues we can go by:
Jade attended middle-school, so she was likely at least 10 years old in Season 1.
Jade is said to be 11 in Sign of the Ox, the novelization based on the TV episode The Mask of El Toro Fuerte.
With that in mind, lets look at how much time passes between the seasons, based on holidays and events featured throughout the course of the show:
Season 1 concludes on Chinese New Year Day of the Dragon.
Season 2, therefore takes place the following year. There are no holiday episodes or major events, so we can conclude Season 2 takes place within the span of a year.
Season 3 features a Christmas episode A Jolly J-Team X-Mas shortly before the season final. This means a year has finally passed since the conclusion of Season 1.
Season 4 features a Halloween episode Fright Fight Night and is followed by a Chinese New Year episode J2: Rise of the Dragons. Therefore, another year has just passed.
Season 5 features Jade's birthday Black and White and Chi All Over. No other holidays or events are featured, though it is likely the episode The Demon Beneath My Wings takes place in October, as the school is taking the kids to see a haunted house.
From all of this, I conclude that one year passes in Season 1, a second year passes between Season 2 and Season 3, a third year passes in Season 4, and Season 5 takes place after Chinese New Year.
If we go by Sign of the OX, Jade is 11 in Season 1. She turns 12 by the end of Season 3. By the end of Season 4, she has turned 13. When she has her birthday in Season 5, she is celebrating her 14th birthday.
Now, Jade does not appear like your typical 14 year old. She is still in middle school, but if she had a late birthday this would still make sense (being 13 at the first semester and 14 by the second semester). But again, she looks closer to a 12 year old, and she is still attached to her stuffed Super Moose. This could be explained away by a couple of things: Jackie Chan Adventures is loosely based on anime, which always makes children super small, and there is absolutely nothing wrong with a 13 year old having a stuffed animal (even though kids act like they are too old for them).
But this leaves me with the question: Did Jade turn 14 in Season 5? Is Sign of the Ox incorrect and/or did the writers lose track of time and intend her to be turning 13 instead? I would love to know what you guys think!