It’s never been an important plot point that Carol has a degree in accouting, but it has been mentioned in passing. Honestly, if Carol isn’t the most mature member of their peer groups she’s in the top 3 at least. She’s more mature than a lot of people I know who are much older than me. Of course maturity and age often get conflated. If nothing else Carol is the more mature of the two of them & Thomas is self aware enough to recognize that much at least.