Question about character ages

In season 1, Georgie is 14. By this logic, Georgie should be 18 in season 5 when he's expecting the baby. However, Mandy says he still won't be 18 when the baby is born.

However, his birthday is said to be in March and in the season 6 episode where Pastor Jeff wages war on Connie's video store, they say it's 1992. This tracks with Basic Instinct being available for rental as it is a 1992 film and would definitely be in stores before the end of the year. Season 1 is set in 1989-1990 so by season 6 it should be set in 1994-1995. I know that the fourth and fifth seasons both end with cliffhangers and pick up where they left off, but it kinda feels like they froze time for Georgie.

Also Sheldon should be 14 by season 6. We know from TBBT that he goes to Germany and loses his dad at age 15 (season 7), so we're on track with him. However, by Georgie time, he should still be 12. Same with Missy and Billy, who are in sixth grade in season 6 (Billy prays to pass an exam because he doesn't want to be held back as he's already the biggest kid in his class).

Anyway, I'm confused and I'd appreciate some clarity on this.