Anyone could risk being illiterate, even as an adult. An affected individual need not necessarily come from a financially poor nation or an environment with poor living conditions. As long as an individual lacks the required education to be proficient in reading and writing, he is an illiterate.
Illiteracy affects many individuals round the globe. Note that some countries face a lower literacy rate than others due to the lack of teaching resources. Some countries may also find no urgent need for a high literacy rate to be present in their country's workforce.
For example, in countries that earn most of their revenue from agriculture, has no urgent need for visual communication to build the economy. Whereas, in countries that earn most of their revenue from tourism and foreign investments, demand a high level of proficiency in registering visual information by the workers, hence in need of a high literacy rate.
Therefore illiteracy could affect anyone at anyplace and is not biased to any social groups. The literacy rate could also depend on the country's resources and need.