This thesis addressed the following research activities: (1) Developing and evaluating the usability of an adult age-independent online assessment tool that includes health care related content and is centered on the needs and capabilities of both older and younger users and, (2) Developing adult age-independent design guidelines. An online tool, which was developed for a previous study, was revised according to the results of the previous study and preliminary development activities based upon the user requirements of older adults (50 years or older). The online assessment tool was also tested with younger adults (between 18 and 35 years of age) to determine the differences between younger and older adults. Subsequent to these activities, a final tool that captured the needs of both older and younger adults was developed and evaluated for usability. According to the usability testing results, a design guideline set for an adult age-independent Online Needs Assessment Tool was developed and revisions were conducted in order to develop the final Online Needs Assessment Tool. The results of these studies, along with the tools developed, provide online survey and interface designers information on older and younger adult user requirements. They provide a knowledge resource for older and younger user healthcare information needs and information about the usability of one online assessment tool to accommodate both older and younger adult users. They also assist interface designers to proliferate and to facilitate more rapid application development / Master of Science