Visible crew/equipment: After Truman sees his father as a homeless man there is a scene where he runs into a vehicle (before the dog runs past). This shot is from the ground looking up. Take a look at the reflection in the passing car, you can see the crew and equipment. (00:15:50)
Visible crew/equipment: When Truman takes his high school sweetheart to the beach, her "father" comes to get her. There is a shot through the father's button cam showing Truman backing away. Look at Truman's shirt and you will see not one but two human shadows. One is the father and the other is the "button" cameraman. (00:25:30)
Visible crew/equipment: In the scene at the travel agents when Truman is trying to book a flight to Fiji the woman asks 'when would you like to leave?', Truman holds up his suitcase and says 'today'. When he holds up his suitcase you can see the boom mic reflected on his suitcase. (00:41:25)
Visible crew/equipment: During the scene following the leak at the plant when Truman's wife is trying to ask him what is going on, she is standing next to a counter. Look at the farthest left door. A shadow of a crew member can be seen moving.
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.