Artie called her a woman, not a teenager. He said, "I assumed, being a woman.....".
You can still be a woman and a teenager and Artie called her both:
|Artie: You're not a girl anymore. Y-y-y- |
Artie: Hmm? I just said that you're a woman! Not, well, you're a woman! Because, you know, you're over s-s-sixteen.
Then later, at the diner:
|I just sometimes forget that underneath the talented computer programmer and fearless junior Warehouse agent, there beats the heart of a teenage girl.|