Continuity mistake: When Fez knocks the clown over and shatters it, you see the head roll in front of the body and out of frame with the nose still attached. Donna then turns around and runs in to take the nose from behind the body of the clown.
Other mistake: When Bob comes to order from the clown, he gets told to go away since they're closed. He starts pulling away, but you can see he doesn't leave the frame quickly. The edge of the car is still visible when the gang is sneaking back over.
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