Continuity mistake: When Red throws the keys to the Vista Cruiser to Eric, you can see he (Eric) catches them with both hands clasped together, then the camera cuts to show Red and Kitty (with the back of Eric's head) you see now that his right hand is clenched in the air, then it cuts again and his hands are clasped together again. He could not have moved his hands that quick.
Continuity mistake: In the scene where Eric gets the keys to the Vista Cruiser, just after he catches them, you can see that the door to the living room is wide open, but before when Red and Kitty were telling him the responsibilities of owning a car, it is closed. No one had gone through the door during the entire scene.
Chosen answer: He isn't. Time moves slowly in the show because they didn't want it to turn into "That '80's Show". In the first episode, he say's he's 17 but the second episode was his 17th birthday so he must have been 16. The first 12 episodes are set in 1976, the next 2 seasons are set in 1977, the next 3 seasons are set in 1978, and the remaining shows take place in 1979 with the final show on Dec 31, 1979 when Eric must be 20 as he turned 17 in 1976.
Myridon