DIFFERENCES in speed of perception and performance between younger and older human adults sometimes increase proportionately with task difficulty, but sometimes remain constant. Gregory1 has argued that the added time is adaptive and has shown how a constant component, such as age-associated rise in ‘noise’ level, could result in a linear increase of decision time. Welford2 suggests that the constant age effect occurs when signals are brief, whereas when perception is not limited, proportionate increases are manifested. We report here an example of a constant difference between age groups in absolute time thresholds followed by an abrupt change in a situation where signals are brief. The implication seems to be that two different deficiencies are operating, both of which could perhaps ultimately be attributed to ‘noise’. Participants in the experiment were required to identify varying numbers of letters and the sudden increase in required time indicates a reduction in the visual perceptual span of older people.