if a researcher is studying the relationship between age and income from wages, and his measure of age is as follows:

Please indicate your age by placing an X in the box to the right of the category to which you belong.

Less than 15

15 to 24

25 to 34

35 to 44

45 to 54

55 to 64

65 to 74

75 and older

At what level of measurement does the variable "age" exist? (Note: ignore potential problems with mutual exclusivity or exhaustiveness for this answer).