The purpose of this study was to determine which factor-age or automaticity of driving skill has a larger effect on the change of driving ability caused by secondary tasks. The experiment was conducted with a group of young drivers in their 20s (25.3 ± 0.9 y) and taxi drivers in their 50s (57.5 ± 5.8 y) with driving experience 2 y (2.2 ± 1.9 y) and more than 20 y (26.1 ± 5.7 y), respectively. The test was composed of a rest phase (3 min), a control phase (1 min), and a task phase (1 min). The tasks of sending text messages or searching navigation aids were selected as the secondary tasks. During the control and task phases, subjects were asked to drive at a speed of 90 km/h and a following-distance of 30 m from the automobile in front. The increase in workload was not significantly different between the two age groups. However, the decrease in driving ability was steeper for taxi drivers than for the young drivers. This show that aging has a greater effect on driving ability than automaticity in driving skill does when this ability is loaded with a simultaneous secondary task.