The types of vaccines given to the older age groups are often done to change the course of the illness/disease once the virus/bacteria has been picked up. For example, to avoid the need for antibiotics and hospitalisation (and death).
There is no point in vaccinating someone if it doesn’t change the course of the illness - hence we are no longer vaccinating immunocompetent younger people against Covid because if they pick up the virus, they are unlikely to become unwell as their own immune systems are able to deal with it.
The human immune system starts to weaken significantly with age, around 65, hence “older” adults need a bit more help. Even if they don’t die, the costs of treating them and the impact on health services is so significant that it makes the cost/benefits analysis worthwhile. You don’t want your cancer surgery postponed because the surgical unit is having to board older medical patients with pneumonia.
The costs and potential vaccine-related side effects (although very small) of vaccinating a whole population of young people against Men B when the biggest majority of them will get no more than a sore throat, are simply not warranted. It doesn’t give lifetime protection - only a couple of years - and young people are likely to live a much longer time. A 75 year old getting the RSV vaccine to keep them out of hospital is unlikely to live for a huge amount of time longer, so waning protection becomes less of an issue.
We’ve got much bigger issues in Public Health such as measles and potentially TB owing to low uptake rates for MMR and mass migration (with TB) now that we no longer vaccinate babies out with high risk communities - the same high risk communities who are more likely to not engage with vaccination programmes.
For TB;
“England is on the brink of exceeding the WHO threshold of 10 cases per 100,000, driven by infections in cities and higher rates among vulnerable, underserved communities”.
https://www.bmj.com/content/392/bmj.s471#:~:text=Once%20considered%20a%20disease%20consigned,would%20want%20them%20to%20be.%E2%80%9D
If I had younger DC and lived in a UK city with a high migrant population, I’d seriously consider getting them vaccinated as protection lasts for decades.