Relation between: advanced age and increased susceptibility to SARS-CoV-1, MERS-CoV, and SARS-CoV-2
I suggest you read this article.
Physical Activity is a Preventive Factor Against SARS- COV-2 in Healthy Subjects (Possible Cellular and Molecular Mechanisms)
Similar to nearly all infectious diseases, as you get older, your immune system becomes weak and weaker. So old people are more susceptible to severe or critical stages of these disease.
From CDC (U.S.): 80% people died of COVID-19 in the U.S. were people older than 65.
Michael Halim Here is a good Nature paper (8/28/2020) for you, entitled " The coronavirus is most deadly if you are older and male — new data reveal the risks. "
*** And, don't forget that the *OLD* and *MALE* died of COVID-19 the most (attached chart from Nature).
https://www.nature.com/articles/d41586-020-02483-2
In my opinion, the receptor of all of them are related to RAAS system.
Please take a look at this useful RG link.
Article Why does COVID-19 disproportionately affect older people?
The full-text of this article is available online.