The Inclusive Design Toolkit is a great resource for product designers. This page link takes you to the section on user capabilities that need to be considered when designing products. It shows how many potential purchasers are left out by not considering universal design principles. Good information is available on other parts of the website as well.
The Toolkit was devised by the Engineering Design Centre at the University of Cambridge (UK) – in Britain they use “inclusive” rather than “universal” design.