。
She was a girl about our age.
Literal
She [topic-は] us [as-と] same-amount of age girl was.
Combines the equal-comparison frame with a nominal predicate. 私たちと同じくらいの年齢 ('an age about the same as ours') uses 同じくらいの to attribute a property nominally to 女の子. The whole noun phrase 私たちと同じくらいの年齢の女の子 ('a girl of about the same age as us') is a stack of の-attributes: each の links a modifier to the next, building up the description from comparison to age to girl. The だった ends in plain past — recounting a past observation.