Special studies were taken up in 1984. Guinea pigs were taken up for experimentation of the vaccine for legionella. To get the apt answer, please check with your doctor

Generally getting the shot once is enough. But at 65, if you had not received in the anteceding 5 years, you might be asked to take a booster. If you are asthmatic or if you are suffering from chronic illnesses or cardiac or kidney or liver related problems or even HIV or spleen problems or if you smoke you might be asked to take a booster even if you are an adult

This is not needed frequently. If you are above 65 or less than 5, you might need it. If you are in between 5 and 64, then you need it in case if your have a low immunity

The shot is good for a lifetime. If you are immuno suppressed, you might be asked to take a booster. If you are reaching 65 years of age, and if you had not taken the shot in the preceding 5 years, you might need to take it up again