We don't know enough yet, but could this become another one of those "childhood illnesses" - like chicken pox? Back in the day, kids didn't get vaccines for that. Often parents would intentionally take their kids for a visit to homes with kids that had the chicken pox, so they could catch it too, get through it easily with mild symptoms, and be immune from then on.
My parents never did that, and when my son got chicken pox when I was in my 30's, so did I, and it was terrible. It was far worse than my son's case, with legions all over. When it developed in my mouth, there was a danger it could swell up my throat and kill me from suffocation.
So it's not like COVID-19 is unique in how it affects different age groups.
COVID-19, though, is an RNA virus, so it might mutate rather quickly, making it resistant to both vaccines and antibodies over time. Imagine if it evolves like influenza. Vaccines would have to target various strains of the virus, with targets missed each season, or it may be impossible to create an effective vaccine as it is for the common cold.
There has also been speculation (and a couple of draft papers, which is all we have at this point) that treatment of serious cases is completely wrong. The evidence for this is the large percentage of intubated patients that die on the ventilator. Far higher in COVID-19 patients than the typical survival rate for pneumonia. Patients are intubated when their blood oxygen level falls too low as a treatment for assumed ARDS. But some doctors say what is happening is more like hypoxia (think altitude sickness). The theory is that COVID-19 is directly interfering with hemoglobin molecules in the lungs, and stripping it of its iron ions. It's not a mechanical problem, which is what the ventilator is trying to treat - it's a chemistry problem. Some doctors are suggesting trying blood transfusions as part of the treatment for serious cases - to supplement the red blood cells that have been damaged with some with functional hemoglobin.
This also explains why we are seeing some success using drugs to treat malaria. Malaria is not a virus - it's a plasmodium. So why would these drugs work on a virus? If the virus is attacking the function of hemoglobin, similar to the way malaria feeds off the oxygen in red blood cells, it makes a kind of sense.
Lots to learn. Everybody practice distancing and stay safe. The experts will figure this thing out.