。
I think she's over forty at least.
Literal
She [topic-は] at-least 40-years [contrast-は] has-exceeded [quotative-と] think.
越えている (こえている, 'has exceeded, is over') expresses that her age has passed the 40 mark. The は after ４０才 adds emphasis: 'forty at least, if not more.' ~と思う reports the speaker's personal judgment. The sentence estimates someone's age from external observation.