Time seems to accelerate with increasing age. This subjective acceleration or compression may be due to the operation of a linear or a square-root function that relates present age to past age. A subjective magnitude-estimation technique was applied in a questionnaire given to 282 university students to determine which function best fitted the data. The results did not support the hypothesis that subjective time is estimated by either linear or square-root functions. Instead, the results indicate that subjective time is estimated with respect to a stable constant independent of age. These results support the notion that the purely subjective perception of time may serve as a measure of psychological temporal processes that remain stable with age. A linear function of age was, however, significantly related to error in the estimation of test-duration.