Onegin’s Tatiana Was Only Thirteen? points out the references to Tatiana's maid's age in reference to her own. Also telling is Pushkin's choice of words:
Pushkin uses the word otrokovitsa. This hard-to-pronounce Russian word is usually translated as maiden but in Pushkin’s time otrok (male) and otrokovitsa (female) referred to children from 7 to 15 years old.
The conclusion that she *could* have been only thirteen seems to be consistent with my memory of Eugene's on-again/off-again conscience and not wanting to betray her innocence. It is possible for a "superfluous man" to do the right thing, even with (or because of) Pushkin's irony.
My concern with this supposition, though, comes from the consistency with Russian societal norms of the time. Would girls of thirteen been allowed to dance with men (as she does on her name-day party)? But then for a work that examines art vs. real life, how much stock should you put in what was common at the time? Like I said, I'll need to revisit this soon. I'm interested to hear from others on Onegin's rejection.