Text (book, webpage or other) written in a font which is so small that it is visible only to those with good eyesight. Those whose age is over forty increasingly lose their close-up vision as they age (known as hyperopia), because their eye-focus gradually becomes more fixed-focus rather than adaptable-focus.
Well, this webpage certainly loves using under-forty-font!
The ingredients on this candy bar are impossible to read in their under-forty-font.