At the age of ( ),he,was a worker.

选项：A:twenty.B:the twenty.C：twenties.D:twenty-first.

A

at the age of ...　　后跟确定的数字（C排除）.不加the（B排除），D错在first如果21岁的话，应该是twenty-one.

你的理解是合理的，但是如果living near boston, and wondered what to do这样是可以的。这样的话，living引导动名词短语修饰主句dave was a worker and wondered about future。 可是句子是 living near boston and ___ what to do. 再填wondered就不对多了，很明显，这里的and连接了两个动名词短语做状语修饰dave was a worker.

At the age of ( 25),he,was a worker.