Why did they make her the age of entering high school, starting in season 10 of Roseanne, and continuing onto the Conners?

Shouldn't she be about 21?

Everyone else seems to have aged appropriately. (Darlene, Becky).

I just thought about that. Harris if I have my facts straight, was born during Season 9 (1996-97) of the old Roseanne show. So if you're going by real world time, she should be at least 25 years old now. For what it's worth, Emma Kenney is currently, 22 years old (she was born in September 1999). Even Wikipedia has pointed out (https://en.wikipedia.org/wiki/List_of_Roseanne_and_The_Conners_characters#:~:text=Darlene%20announced%20she%20was%20pregnant,of%20season%2010%20in%202018.) the apparent lack of continuity.

Last seasons have been retconned, and weren't taken into account.
Authors so took some liberties.

Since the Conners started only one that doesn't seem to age appropriately is Beverly Rose =D