This study examined the effect of age on descriptive walking-speed instructions commonly used in health promotion. Participants were 9 young (20–23 years) and 9 older (75–83 years) women. Oxygen uptake and walking speed were measured in response to descriptive walking instructions ("slow," "comfortable," "brisk," and "fast"). Although the older women walked »20% slower in response to all walking instructions and with significantly lower oxygen costs for brisk and fast, the intensity of the exercise represented a much greater percentage of VO2max and showed greater interindividual variation. When asked to walk at a brisk pace, the older women averaged 67% VO2max (SD 20.6), whereas the young women averaged only 45% VO2max (SD 4.5). With older people, brisk might elicit an exercise intensity unnecessarily high for physiological benefit and that might compromise safety and adherence, which emphasizes the need for validation of carefully worded exercise and training guidance for older adults.