It’s not about the people it’s about the system being able to cope, older people will need more care as they are more likely to suffer the worse symptoms, therefore increasing demand for teams of health care professions and resources, and we will run out of both. It’s also not about physical fitness, all our immune systems peak in our twenties, after that were all slowly deterioracting. Yes some are crumbling faster than others, however, the ability of a 70 year old to recover from this has been proven to be several magnitudes worse than that for a 30 year old, and that is with access to the right Levels of care.
There is no point trying to stop the virus until there is a cure/vaccine, as I said earlier there are two ways of dealing with this, a blanket close down society to reduce spread or targetted measures that cause some inconvenience but keeps the most vulnerable safest the longest.
Its about choosing the right pathway to minimise the number of deaths whilst maintaining as much of normality as required.