f’c is the specified compressive strength of concrete using standard cylinders of six inches diameter and twelve inches height. Usually this is prescribed at the age of 28 days.
The word specified means here usually a set of guidelines meeting the minimum requirements.
In actual construction practice, the dead load on the flexural element comes quite early (10 days or so). Therefore, it has become a kind of practice to associate concrete strength at 28-days. However, it must be kept in mind that the full service load on the lowest column of a high rise reinforced concrete building may require months or years in some places. Therefore, it should not be surprising if concrete strength for such column is specified at later ages (56, 90 or 120-days).