Molly Aged more than can be accounted for between books 8 and 9 (Proven Guilty and White Night)
Per the "Timeline Chick"
We haven't quite pinned down specifically what years these events take place, so let's say "Storm Front" and "Fool Moon" take place in the year 0, and every thing else is either BSF (Before Storm Front) or ASF (After Storm Front).
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12 BSF: Molly Carpenter is born, according to WN and beyond. She's suddenly a year too old in WN, and the change is kept consistent for the subsequent stories.
(11 BSF: Molly Carpenter is born, according to DM and PG. She's suddenly a year too old in WN, and the change is kept consistent for the subsequent stories.)
Here are the relevant book quotes:
"You're sixteen"
"Seventeen," she said, with sparks of indignation and another thick "s."
"Whatever," I said. "You're a juvenile. You should call your parents."
I stripped the glove off and held it up, in my fingers spread. It didn't look as horrific as it used to, but it was plenty ugly enough to make an impression on a nineteen-year-old girl. "This isn't a goddamn movie, Molly.
Molly's age can be explained by the fact that some of the early books got published in a different order than written. Priscellie covered this at one time IIRC.
With that note, I'll put in a Doylist explanation of that "Discrepancy."
According to the following WoJ, the order for DB and PG got switched from their planned sequence. Since the discrepancy circles around how much time passed between PG and WN with respect to Molly's age, it can be assumed that for the overall plot, Molly was supposed to be Harry's apprentice for a few years WN.
You planned out the 20 books and apocalyptic trilogy for the books and how much have you changed now that you have gotten into it?
Not a lot. I changed around the events of Proven Guilty and Dead Beat because Proven Guilty, the part with Molly was originally going to come first and I told my editor, "This one is going to be a little quieter, a little bit more personal than the one before, it's going to be focusing a little bit more on just a couple of people and stuff that's important to Harry." and my editor says, "well, you know, that might not be a good idea for this one." And I'm like "What?" "You might want to have a story that's a little bit bigger" "What?" "You might want to have a story that's a little bit broader and thicker. Something that people can really get their teeth into. Something that's going to be a little heavier and have more weight." I'm like "Wait a minute are you saying I'm going into hard cover?" She's like "That's not what I'm saying... I didn't say that!" Cause it was a surprise for me I suppose. So then I said, "Ok we've gotta go with zombies and ghosts and animated T-Rex for that one then."
Edit: I'll note here that when Priscellie confronted Jim about this at a signing, he recruited her to be a Beta. Something similar happened to Terri Bane when she punched Jim's shoulder at an interview for letting Harry not check up on Mouse's shoulder after the dust settled during White Night.