Okay, I'm gonna go ahead and preface this by saying the creator of Ed, Edd n Eddy has sadly already spoiled the fun a bit and debunked this popular fan theory. I'm going to show it to you regardless, though. In essence, the theory is that the the kids that inhabit the Cul-de-sac are actually dead, and that this entire show takes place in a purgatory-like setting.
The main things it points to are the fact that the children have green and blue tongues (the tongue is known to turn a bluish shade when you die), there are literally never any adults around, the summer seems endless (especially in early seasons), and they're in a small, isolated cul-de-sac bubble that makes up their entire world. Of course, take everything here with a grain of salt, but there are more reasons that help make a compelling argument for our boys and their gang being dead as hell.
If you buy this theory, the image above will help illuminate things. The theory states that the children of the cul-de-sac actually come from different time periods - ranging from the early 1900s to the beginning 2000s. If you think about it, it's very difficult to ascertain what time period all of the show's episodes happen during. You sometimes catch a glimpse of an adult every now and then, but it's rare and always quick.
Rolf is first in the theory, coming in the early 1900s. His family apparently moved to Peach Creek to establish a farm on its land. So, Rolf dies in 1903 when working on the farm and getting trampled by farm animals. This was the supposed reason for his one cow, one goat, a few pigs, and a few chickens in the afterlife.
Second is Johnny 2x4, who came to the cul-de-sac not too long after Rolf's death. He had no friends besides his imaginary created friend Plank, and after dying in 1922 from a long battle with tuberculosis he joined Rolf in the afterlife with Plank, who was the last person/thing he saw in his life.
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Next was Eddy. He was born in NY but moved to Peach Creek during the Great Depression era (1930s-ish). He was on the look-out to stay alive and make a "quick buck" often, so he constantly would set up scams to get money for the cul-de-sac kids. When he was in the middle of one of his schemes, the kids from the cul-de-sac chased him down to a lake (presumably after he tried to run a scam on them) and he jumped into it. He drowned and passed in that lake, and continued his swindling in the afterlife.
Ed and Sarah were next. Their father died fighting in WWII, and as a result their mom sort of became disconnected, aloof, and distant. Sarah picked up a bossy attitude and persona, trying to fill in for the roles their parents had previously had in their lives. Ed had a bit of a different coping mechanism. He shut the world out and delved into nerdy things like comic books and monster movies in order to escape an unhappy life. They both died in a freak car accident in 1953, and then joined the past kids in purgatory.
Nazz was born in the 1960s with hippie parents. She was a flower child, free-spirit type and in the summer of '79 a serial killer escaped from an asylum and - bare with me here, it gets weird - raped and murdered her and her entire family. Because this event was so traumatizing, Nazz (like Ed) shut the world out and entered into the Purgatory. This explains why she's not shown working around the house like the other children would.
Edd with two Ds was born in the '80s and was raised by very strict and controlling "helicopter parents." They always pushed him to succeed academically and be cleanly, neat, and presentable. He is thought to have died from a gas leak causing an explosion with the bunsen burner from his chemistry set.
Kevin was born the same day that Double D (Edd) died. He was born to a broken-down house and lived with an abusive father. Because of his situation at home, Kevin would act out on the other children of the cul-de-sac, becoming a bully. They say that "hurt people hurt people" and this is one of those instances. Tragically (and once again, this gets brutal here) he dies after getting beaten by his father in a drunken rage. He died on the way to the hospital, and entered into the afterlife with a re-imagined version of his father as loving. He's still a bully though...
Jimmy was the last kid, born in the 2000s. He was diagnosed with leukemia (more tragedy, I know, sorry). He was never associated with the rest of the gang. He ended up dying young and ending up in the Purgatory.
The Kankers were different from the neighborhood kids. They were thought of as being demons. They were the only characters with normal-colored tongues as they didn't cross over to Heaven. They are attracted to the Eds, which is probably because of the fact that they are the weakest willed children of the cul-de-sac. Another theory is that they each symbolize a certain Deadly Sin.
Like I said before, this theory, while interesting, was debunked by Danny Antonucci, the creator of the show. It actually aired from 1999–2009 which is pretty impressive. Quality show.
(Source: FanTheories Wiki)