Oh, Baby We Got a Good Thing Goin' - S7-E18
Continuity mistake: When Kelso is trying to get the baby to not cry, you can tell Bob covers its face to the camera so we don't see that it's not actually crying. Cut to a different angle, and the blanket surrounding the baby has moved. Cut back, and the blanket is covering the baby again.
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