In World War II if you were between 25 and 50 you could help England by joining one of the barrage balloon squadrons of the auxiliary air force. It said so on the poster.
Barrage balloons were bags of lighter-than-air gas fixed to steel cables that were anchored to the ground. They could be raised or lowered by means of a winch, often attached to a lorry for increased mobility, and their purpose was simple, ingenious, and effective: To fill vulnerable air space with bulk, thus stopping enemy aircraft from entering it at low levels. This meant that enemy aircraft were forced to fly at higher altitudes decreasing bombing accuracy, making the aircraft more easily spotted, and making it simpler for the ground based Bofors anti-aircraft guns and fighter pilots to attack them.