How can this as~as structure be parsed?

Question

Did she have a precursor? She did, indeed she did. In point of fact, there might have been no Lolita at all had I not loved, one summer, a certain initial girl-child. In a princedom by the sea. Oh when? About as many years before Lolita was born as my age was that summer.
(Lolita, Vladimir Nabokov)

I can see the as~as expression, two adverbial: ‘before Lolita was born’ and ‘that summer’ in the highlighted part. But with this I can’t understand the phrase at all. Would you show me the meaning with parsing?

Answer 1

As Bill Franke discerns (and solves), it's a math puzzle—Vladimir Nabokov channeling Lewis Carroll.

When is understood as in what year. My age ... that summer is equivalent to the number of years after I was born, and the *as ... as ... * construction states that this is equal to the number of years before Lolita was born:

The year in which that happened (let's call it Y_X) was as many years before the year in which Lolita was born (Y_LB) as after the year in which I was born (Y_HB).

That is, the year in question fell midway between the two birth years:

Y_X = Y_HB + ((Y_LB-Y_HB)/2)

Following Bill we read the book and discover that HH is 25 years older than Lolita:

Y_LB-Y_HB = 25

By substitution, then:

Y_X = Y_HB + (25/2) = Y_HB + 12.5

The event occurred in the summer when HH was 12½ years old.