Why do some people present no symptoms? Does it have any relation to age or preexisting health conditions?
We don't know. It could be the particular virus is less virulent in the person who is asymptomatic. Not every person is infected by the exactly the same virus. All viruses mutate which can increase or decrease the virulence of the virus. Also, every person is different genetically. We will talk about how SARS-CoV-2 infects cells through a particular protein called ACE-2, but ACE-2 can vary between people. Age and preexisting health conditions predispose to infection and response to all pathogens, not just to this virus. Our immune system ages as we age. If someone already has a disease, then the body is already compromised.