This is a proposed/tentative answer only...
Statements from official public health sources could be summarized as: older people and those with certain pre-existing health conditions including compromised immune systems are more likely to be more severely affected.
"More severely affected" means requiring hospitalization, suffering permanent lung damage, or dying. "Requiring hospitalization" may be equivalent to a death sentence if the health care system is so overwhelmed that not all who need the necessary care can receive it.
"More likely" means that younger or otherwise healthy individuals are by no means certain to recover from an infection free of any long-term consequence... they are just more likely to.
There are already reports in the media of younger, otherwise healthy individuals dying as a direct result of COVID-19 infection. There doesn't yet seem to be enough known about this disease to make any reliable predictions about the effects of an infection on any given individual; it can at best be expressed as a probability.
Regardless of age or health status, deliberately allowing people to become infected knowing that as a direct result, some fraction of the population will suffer permanent lung damage or die is not ethically sound policy. There can be no guarantee that isolation measures will be good enough, or that there will be a high enough level of public compliance to protect the safety of the vulnerable, so it will place the vulnerable at risk anyway.
The whole point of immunization vaccines is to do the same thing as you suggest, but without suffering the effects or consequences of the disease. The problem is that it takes time to develop, mass produce, and distribute a safe, effective vaccine targeting a new virus. The whole point of social distancing is to slow the spread of a disease in order to minimize the harm it will do, and if a vaccine is ultimately coming, buy the time it takes for that be accomplished. The other possible outcome from social distancing is that the virus could eventually "burn out" - when all infected individuals recover and and become non-infectious before they have an opportunity to infect anyone else.