Given her actual answer, one would hope it was not taking into consideration some of the facts Emily in Paris’ writers had attributed to Emily, including that Masters Degree.
So, even if she’s not up on corporate advancement, she’d sort of be able to figure out Emily Cooper’s age.
Lily Collins is an actress who did attend college at the University of Southern California, though she told Shondaland in 2019 she hadn’t graduated yet.
Basically, though, if Collins had forgotten about that bit and doesn’t have a ton of experience with office advancement, it’s easy to see how she could have imagined Emily Cooper as younger in her mind.