That is, unlike the carbon-only ring like in benzene and napthalene, there are other elements in the backbone, such as in pyrimidine and purine.
... the age of 18, 5.7% from 18 to 24, 31.9% from 25 to 44, 23.3% from 45 to 64, and 13.9% who are 65 years of age or older. The median age is 38 years. For every 100 ...