Like most of the general public, Jon is under the impression that Seras is an ancient inhuman creature who operates on her own unknowable logic and does not comprehend the ways of mortals.
He has no idea that she was only turned two years before Stephen…and that, even taking into account the fact that her body hasn’t changed since, she’s more than a decade younger than either of them.
Seras: So he’s under your control?
Jon: Well, I think “control” is a bit much…
Seras: Do you also take responsibility for the man I killed?
Stephen: I told you, that wasn’t Jon’s fault! Except in the sense that he is tiny and increasingly frail.
Jon: I never told you I was sick…
Stephen: Oh, like I couldn’t smell it? I know you’re naturally decrepit, but even so!
Jon: Oh, no, she’s giggling. Seras Victoria giggles?
Seras: It’s all right. You just reminded me of someone.
Jon: Do you mind? I know you’re not human, but for those of us who are, this really isn’t funny!
Stephen: Jon! Let the world’s most powerful vampire laugh!