Generally it isn't on the individual to try to weigh something like that up. Here in the UK, at least, the Vaccine will be offered to people in order, so if you are offered it, there would be no reason to defer—others who need it more should have been offered it before you and you accepting won't deny them anything.
This is why countries and public health departments are discussing and agreeing on who gets a vaccination first - people in care homes, front-line health workers, or perhaps even young, healthy people because they heavily drive the spread. I don't think there's a situation anywhere where it's a free-for-all.
Personally, I will take the vaccine as soon as it's made available to me. Even if my risk is perhaps low (which may or may not be true), taking the vaccine reduces the chance that I would spread COVID to someone else. It's a collective effort to reduce the spread, not an individual or selfish one.
it would prevent infection and transmission as much as any other vaccine would. you're just priming your immune system to handle something its not actually seen before. you can still get the virus, it will still affect you, but your immune system will recognize and deal with it much much faster. thinking logically, if you are given the virus, your body will still make copies for a short amount of time and you would still be able to give it to someone else. getting the vaccine makes the window of opportunity much smaller and dramatically lowers the risk that you will die from it.
The answer is still yes for both cases, long term covid damage is bad at any age and so is taking out a grandparent, co-worker you like.