Fortunately for us, Lynn had Michael make a statement that can reconcile his and Elizabeth's being born in 1974 and 1979 with their graduating from High School in 1994 and 1998 respectively. That's because in his last retcon, he'd tried to explain away the Dreaded Hybrid by telling us that while the Pattersons would go on living their lives in private, the artist who told their story was no longer doing the draw-cumentary she did so well. Given that we're seeing is meant to be the saga of the Pattersons as told to Lynn Johnston, it might be that Elly and John are doing the same damned thing that they condemn Mira for: massaging the truth to look better.
The terrible secret that they might be at pains to hide from the public is that owing to being fairly stupid, Mike and Lizzie had to repeat a couple of grades in elementary school. It isn't very hard to imagine that a boy who simply cannot wrap his tiny brain around basic arithmetic and a girl who is so stunned and stupid that common-place sayings that she hears every day sail over her empty head might need to repeat a year or two so that someone can make sure that they actually learn something. It also makes sense that someone like Elly who, despite being an idiot in her own right, is at pains to boast about her love of learning, doesn't want to admit that her children are as dumb as she is. Thus, she ignores the years that 'didn't' count and states that her wanted children were two years younger than they actually are.