A mental-rotation task was presented to young (18-28 years) and old (60-76 years) adults to simultaneously assess age-related changes in performance, response monitoring and adaptive behavior. Relative to young participants, older adults were less inclined to adjust their speed at the expense of accuracy. They displayed a larger number of slow errors, smaller error potentials (Ne and Pe), more immediate corrections of errors when detected, and a larger speed reduction on trials following an error. The data suggest that for older adults an increase of task complexity sometimes caused a radical failure in determining the correct response, rather than a gradual reduction of efficiency.