Can someone explain how Boris is now in ICU but the Prince recovered so fast even though he is much older
For the same reason that one if my children had chickenpox with half a dozen spots and no other symptoms while another was covered head to toe, had a fever, couldn't eat, and ended up hospitalised - viruses affect different people in different ways.
Some people will hardly be unwell at all and will bounce back quickly, others will be very unwell and will need medical support, some will sadly die, and others still will fall somewhere in-between. You don't know which person you will be unless you catch it.