I recently finished watching every episode of South Park in order of air date. Whilst watching the episodes, I took note of how old the boys were mentioned to be. I have created a table that displays how old the boys were when said episode took place.

Episode Age
Volcano 8
An Elephant Makes Love to a Pig 8
Starvin' Marvin 8
Damien 9
Tom's Rhinoplasty 8
Cartman's Mom is Still a Dirty Slut 8
City on the Edge of Forever (Flashbacks) 8
Clubhouses 8
Spookyfish 8
Merry Christmas Charlie Manson! 8
Gnomes 8
Sexual Harrassment Panda 8
Hooked on Monkey Fonics 8
Are You There God? It's Me, Jesus 8
Cartman's Silly Hate Crime 2000 8
Cartman Joins NAMBLA 8
Probably 8
The Wacky Molestation Adventure 8
Scott Tenorman Must Die 8
Cartmanland 9
Proper Condom Use 8
Kenny Dies 8
Asspen 8
Freak Strike 8
Professor Chaos 8
The Simpsons Already Did It 8
Bebe's Boobs Destroy Society 8
A Ladder to Heaven 8
The Biggest Douche in the Universe 8
Toilet Paper 8-10
I'm a Little Bit Country 9
Raisins 9
Good Times with Weapons 8
Up the Down Steroid 9
The Passion of the Jew 8
Pre-School 9
Best Friends Forever 8
Marjorine 9
Bloody Mary 8
Tsst 8
Cartman Sucks 8
The Snuke 8
Night of the Living Homeless 9
Imaginationland Episode I 8
Over Logging 9
Pandemic 2: The Startling 10
Butters' Bottom Bitch 8
W.T.F. 9
Whale Whores 9
You Have 0 Friends 8-9
201 9
Mysterion Rises 9
Crack Baby Athletic Association 10
You're Getting Old 10
A History Channel Thanksgiving 9
Butterballs 10
I Should Have Never Gone Ziplining 10
World War Zimmerman 9
The Hobbit 9
South Park: The Stick of Truth 9
Cock Magic 9

I believe the age displayed on the fourth graders' wiki pages should be lowered from 10 to 9; the latest mention of a fourth grader's age was Kenny's in "Cock Magic", where he was said to be nine years old. He was previously mentioned to be ten years old in "I Should Have Never Gone Ziplining".

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