Well, Dean may not have done the math because they clearly went back to a different month of 1978. They left January/February of 2010 and had to be in late May or June, possibly July, of 1978 (given that Mary knows she's pregnant but isn't showing yet). Dean was conceived in April of 1978 (unless he was really premature--40 weeks puts conception on April 19, 1978). Given that the boys weren't exactly looking at newspapers and caring too much about the exact date, he could easily have not realized it was late enough in the year for Mary to be pregnant yet. Also? Not really one of those thins that you think about.

John and Mary did age really fast in 4 years, but they were raising Dean Winchester in those four years. (Also? Actually a little more than 5, but still.) LOL. I do have an issue with the age difference between the actors as well--it worked when it was 10.5 years before the beginning of Pilot, but now it's only 5 and a few months.