Question

3 reasons why older people are more susceptible to lung cancer?

Answer 1

well as people get older their internal machinery (that replicates DNA and makes mRNA and in turn proteins (i.e. enzymes)) degrades and makes mistakes more and more often. A single error could affect the enzymes ability to perform it's function. Also, whole cells can be affected during the DNA replication process, not just enzymes. This can apply to all kinds of illnesses not just cancer though.

You could also say that indoor smoking was legal until very recently, and older people have more years "under their belt" in these 2nd hand-smoke filled environments, which everyone knows causes cancer.