Continuity mistake: When the fallen lamp has settled, the glass and parts of the housing are in the sunlight, and the light is in the shadow, with the camera looking into the side of the shadow. As Truman comes out to investigate, the light has been turned to a new direction, the pieces of the housing are missing, and the glass is splayed out a lot more than it originally was.
Visible crew/equipment: Right as Truman is leaving to go to Fiji in the sedan, he backs out of the driveway. The shot is a camera attached to the rear of the vehicle. The rig is reflected in the bumper on both sides of the license plate.
Chosen answer: Clearly, in his media interviews, Truman's best friend, Marion, indicates that he did, indeed, grow up with Truman since they met in elementary school. One could extrapolate that his classmates did, as well. A sudden change of actor from one day to the next would be too much of tip-off to Truman. As actors leave the show, there is a story along with it. His father is presumed to have died, and returns later. I'm sure others move away. The main characters (Truman's wife, parents, close co-workers, and other key figures in his life) also appear to live "in world" and have devoted their entire lives to the Truman project. It is not made clear about the extras, though.
Michael Albert
It's kinda hard to believe though that a little boy (Marion) is faking it all so well for all these years since his childhood without raising any suspicion.
He probably didn't get to know it until a late age. In the bus, a little girl was confused and almost tell her mum that there was Truman, so a kid can't keep up the secret. I'm pretty sure they don't know it. But on the other hand, when he says he wants to be an explorer all the classmates freak out, as if they knew it would put in danger the show. So maybe the classmates and even the best friend had been selected after making sure they wouldn't mess up and reveal the secret.