At this point, their 12-year age gap is a major plot point, especially when her parents find out.
The retcon essentially de-ages her by roughly 1.5 years.
If you are watching the show and find yourself asking, "Wait, how old is Mandy supposed to be?" you aren't alone. The age gap between Georgie and Mandy is a central plot point in the series.
Mandy is closer to Mary’s age than Georgie’s age : r/YoungSheldon
