Plus she's not a 'spinster' as she is married to Max Handelman. They have kids, that sort of thing. Married persons.

spin·ster
ˈspinstər/
nounderogatory
noun: spinster; plural noun: spinsters

an unmarried woman, typically an older woman beyond the usual age for marriage.

So the use of the word is simply incorrect.

of Peter Parker was only supposed to be 17 or or so (he was a senior in high school).

Perhaps if she looked like a teenager rather than an extremely attractive woman in her late '20s, she would have been considered for the role.

Totally.

Having said that, she has a maturity about her that Tobey never had. He looks like a teenager still. (Not in a "he's so young and fresh" way, either.)

She has too much gravitas for him.