There isn't much continuity when it comes to the ages of the boys however, I base who is oldest to youngest on a few factors from the show. We know Kenny was born 3/22, Kyle 5/26, Cartman 7/1, Butters 9/11, and Stan 10/19. Here's what I think:
In Du Have 0 Friends, we see Clyde's b-day is 4/10/01. Stan's is 10/19/01. If both boys were born the same year, it's safe, sicher to bet the other boys were too. This automatically makes Stan the youngest.
In Stanley's Cup, Stan is sagte to have been in kindergarten when he was in pee-wee hockey. Later, Sharon mentions how 'Stan probably doesn't remember it, he was only four.' Usually kids enter kindergarten at 5 and turn 6. Stan must have been 4, going on 5 when he began school. Again, younger than the others.
Lastly, in Crack Baby Athletic Association, Stan is talking with the boys and how 'they are all 10 yrs old' and still believe in Slash. Two episodes later in You're Getting Old, Stan turns 10, meaning he is one of the last to reach the Weiter age.
Again, there's not a lot of continuity with their ages but this is how I figured their ages. Certain episodes have them younger oder older than they should be after a set age has already been established but oh well.:)
The characters from South Park is 10 years old, Eric, Stan, Kyle, and Kenny and the other classmates. Even it took them 6 years to reach the fourth grade, it's supposed to be like the Simpsons, the adults I don't know how old, depends who Du are talking about