Who catches whooping cough?
Answer: Mainly teens and adults
Most cases in the western world are in teens and adults because children up to that age are well protected by the shots they get at a young age.
It all depends on the environment you live in. Medically and economically developed, or otherwise.
Where there is no immunization against whooping cough in a population, most will have had the illness by the time they are five years old. Immunity after natural infection is thought to last perhaps 10 years or so but nobody is sure because immunity probably gets boosted by reinfection that may cause no symptoms at all.
In the developed world we now live in an environment where most children are immunized early in life against whooping cough, giving them important protection against it at an age when it would otherwise be so easily spread by them to their newborn unimmunized siblings whom it could kill. We must remember that immunization has drastically reduced the impact of whooping cough on our populations, and giving a booster in pregnancy can prevent babies dying of it very effectively.
So nowadays in developed communities there are three groups of people who are susceptible.
It used to be children under 5 who caught it before about 1950. At the present time (2019) in the UK where the immunization rate in children is about 94% (2011), official figures show that although most cases are in the over fifteens, ie adults, the first year of life is the year of age it is most common in.
It is very likely the situation in Australia, New Zealand and North America is similar.
It needs to be noted that as well as what we can recognise as whooping cough, pertussis bacteria can also cause a milder form of coughing illness that can be very similar to milder coughing illnesses such as caused by viruses. Modern antibody testing has suggested that in school and university age children perhaps 6% of coughs lasting between 2 and 8 weeks can be due to Bordetella pertussis, without is taking on the recognisable paroxysmal nature. Pertussis bacteria can also infect people with no, or minimal symptoms and they can get immunity thereby. The exact significance of this phenomenon is not known for sure.
This field is being investigated thoroughly and may result in improved vaccine strategy in the future. One consequence is that less developed countries are being urged to not change from whole cell vaccine to acellular types.