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I figured she was at most 30 years old.
Literal
She [topic-は] at-most 30 years-old is [quotative-と] I [topic-は] thought.
Two topic-marked phrases coexist: 彼女は inside the embedded thought, 僕は in the matrix clause. This nesting is fine in Japanese — each topic scopes only to its own clause. 僕 (boku) is a casual-to-polite first-person pronoun; the choice signals the speaker is male and that the register is informal-friendly. Compared to the more formal 私 (watashi) or the rough 俺 (ore), 僕 sits in the conversational middle. The せいぜい... と思う frame ('I thought she was at most...') is the standard way to express an age estimate that turned out to undershoot reality.