In its Universal Declaration of Human Rights, more than 50 years ago, the United Nations declared literacy to be a human right. (...) One-fifth of people 16 years of age and older are not literate. To focus attention on the 861 Million adults and 113 million children who do not have access to literacy, the United Nations declared a Literacy Decade from 2003-2012 to focus particularly on adult literacy. 
Illiterate means never having learnt to read or write. Usually illiteracy means having little or no formal education. 
If a well educated person loses the ability to read and write, for example through going blind that person isn't called illiterate.
Learning to read and write at 90
Priscilla Sitienei is a Kenyan Woman who is roughly 90-year old  and never had the chance to learn reading and writing as a Child. She is now probably older than any other primary school pupil in the World, others call her "Gogo" (grandmother) affectionately. Priscilla Sitienei wants finally to be able to read and write. More importantly she is encouraging others in her village to get an education and has improved her School a great deal.  
References and footnotes
- Adult Literacy
- 10 Countries With the Worst Literacy Rates in the World
- Accurate birth records weren't kept in her time.
- Kenyan grandmother at school with her great-great-grandchildren
- Oldest student?