、。
She's an adult, so we should treat her accordingly.
Literal
She [topic-は] adult is [because-だから], that befitting-to treat [should-べきだ].
Three constructions stacked. だから is the contracted form of だ + から ('it's X, so'), the casual reasoning connector — slightly stiffer than から alone in this sentence-initial role. それ相応に combines それ ('that') + 相応 ('befitting, appropriate') + に (adverbial) — 'in a manner befitting that,' i.e. 'accordingly.' べきだ is the moral-obligation 'should' — stronger than ~たほうがいい ('would be better to') but softer than ~なければならない ('must'). Together: 'given that she's an adult, the appropriate response is...'