It is more crucial than ever to consider the elderly when designing products. One of the important things to remember is the legibility of characters, since this influences the accessibility and usability of products. We studied the effects of character attributes on legibility using 70 subjects (60 aged 46–79 with no apparent ocular problems except presbyopia, and 10 aged 68–80 with cataracts). Thirty 10-key television remote control models were made. The characters on each were different, and were based on combinations of three attributes—height (5 levels), thickness (3 levels), and display mode (2 levels; positive: black characters on a white background, negative: white characters on a black background). Participants subjectively evaluated the legibility of the characters for each 10-key sample under two ambient illuminance conditions: 500 lx and 50 lx. They also performed paired comparisons on samples with the same height and thickness but different display modes. Subjective data were analyzed using a three-way factorial ANOVA for character height, thickness, and display mode. Legibility was significantly affected by all factors for those without cataracts under both illuminance conditions, and the interaction between thickness and display mode was significant. A two-way factorial ANOVA was performed for height and display mode for each thickness. Legibility was, in general, better for thinner characters in negative mode and was better in negative mode under dark illuminance for those with and without cataracts. This tendency was more pronounced in the elderly (over 65 years old). Paired comparisons showed that legibility was better in negative mode under both illuminance conditions, especially with medium and thin characters, for those with and without cataracts. Although there are few accounts of the effects of positive and negative modes in various existing guidelines, this finding would suggest that legibility can be enhanced by using the negative mode.