He was scarcely sixteen when he joined the army.

他参军时刚刚十六岁。

He wished to join the army,but was rejected as medically unfit.

他想要参军,但因体检不合格而未被批准。

Five months have elapsed since he joined the army.

"自从他参军以后,已经五个月了。"

"According to the law, a young man should enlist when he is 18."

根据法律规定青年人年满十八岁必须参军。