Many people become unable to take care of themselves as they age.

Literal

Age [as-につれて] self [possessive-の] care [object-を] see-can-not become people [topic-は] many.

面倒を見る ('to take care of') is a set phrase. ~られなくなる stacks potential negative + なる: 'come to be unable to.' ~につれて marks proportional decline.