The further you are away from existing or understood user-behavior, the steeper the learning curve. While the ability of people to handle new and unfamiliar experiences varies—early adopters being very accepting of the unfamiliar, while the elderly being very averse—there is a point for everyone where the unfamiliarity of a product makes it unusable.

Screen readers fall beyond this point for many blind users. While they may be usable for those with some computer knowledge, for those who are losing their vision from age related conditions, the complexity of the software can be too much to bear, even with special training. 

Tools that are designed for the elderly should have as shallow and short a learning curve as possible. Not only that, but devices seeking to establish an entirely new product category, like Google Glass or the iPhone before it, should be as similar to current user behavior as possible. iPhone did an amazing job of this, taking advantage of the prevalence if iPods and cell phones; iPhone were just an iterative step. Google Glass on the other hand is a much more alien experience.

Screen readers and Google Glass may seem like two very different products, yet they share a similar distance form current user behavior that they may not be able to ever be attractive to an über-tech-savvy audience.