。
How old do you think she is?
Literal
She [topic-は] how-old is [quotative-と] (you) think [question-か]?
An embedded question + a question of the listener — the two-step structure Japanese uses for 'what do you think X is?' Inside the embedded clause, 何歳だ ('is how old?') is the bare question; the と attaches that question to 思う as its content. The final か turns the matrix sentence into a polite question to the listener. Note the absence of an explicit subject for 思う — it's understood to be the listener (you). This embedded-question-with-と-思う frame is a building block for asking opinions and guesses.