There are many types of sciences, the two focused in radiometric dating are operational science and historical science. Operational science is a systematic approach to understanding that uses observable, testable, and repeatable experimentation to understand how nature commonly behaves. Historical science is interpreting evidence from past events based on a presupposed philosophical point of view.
Radiometric dating uses both of these types of science (measuring the amount of daughter element in the rock sample is an example of operational science, the conditions present when the rock formed can only be studied through historical science.)
Since radiometric dating uses both of these types of science, we can't directly measure the age of something. A combination of scientific techniques of the present and assumptions of historical science would be needed to estimate the age. There are three critical assumptions that can affect the results during radiometric dating: