Generally speaking, age is an ordinal variable since the number assigned to a person’s age is meaningful and not simple an arbitrarily chosen number/marker. Nevertheless in certain situations where the age is not specified, for example, when we classify people as children (below 18) or adults (above 18) then age can be considered as a nominal variable.
Why is age an ordinal variable?
Recall that by an ordinal variable we mean a variable that allows us to “rank” the data or assign “order” to the given data. This can clearly be done when we are given the ages of various individuals. We can rank individuals in order of their age beginning with the oldest person and ending with the youngest person among a given group of individuals.
Note that the exact number assigned to the age of a person is in some sense still arbitrary. For example, in some Asian countries like South Korea when a child is born the child is considered to be “one year old”. This is in contrast to the general international age system where a child is considered to be “one year old” after a year has passed since the birth of the child. Notice that in both systems of assigning age, the concept of ranking older and younger individuals is still objective and not arbitrary.
When can age be considered as a nominal variable?
In many countries, individuals are aclassified as adults if they are above a certain age (usually above 18) and non-adults if they are below that threshold. In these kinds of classification, the exact age of the individual does not matter. For example, if we are given the ages of 10 individuals we may assign the number 1 to the adults and the number 0 to the non-adults. Then the sequence 1100011111 means that there are 7 adults and 3 children in the group of 10 people. Note that the numbers “0” and “1” are purely arbitrary placeholders. In this case we may consider age to be a kind of nominal data.