* Proto-writing: The earliest forms, like the use of tally marks to record quantities (around 30,000 years ago), are considered precursors to writing, not reading in the way we understand it. These are more symbolic representation than a system for conveying complex information.
* Pictographic writing: Around 8000 BCE, systems emerged where symbols represented objects directly (think of a picture of a sun to mean "sun"). This represents a significant step toward reading, as interpreting these symbols is a form of reading, albeit a very basic one. Examples include early cave paintings and Mesopotamian tokens.
* Logographic writing: This developed later, where symbols represented words or morphemes (meaningful units of language). Sumerian cuneiform (circa 3200 BCE) is a prime example. This is closer to what we think of as "reading," as it involves deciphering symbols for meaning, even if the symbols don't directly represent sounds.
* Syllabic writing: Around 1500 BCE, scripts emerged where symbols represented syllables (like "ba," "ka"). This simplifies reading compared to logographic systems. Linear A is an example.
* Alphabetic writing: Around 1050 BCE, the Phoenician alphabet emerged, representing individual sounds (phonemes). This is the ancestor of most modern alphabets and represents a major leap forward in the ease and accessibility of reading. The invention of the alphabet made reading far more widespread and efficient.
So there's no single "when." The transition from pre-writing to full-fledged reading was a long and complex process spanning thousands of years, with different writing systems emerging at different times and in different places. While proto-writing might stretch back tens of thousands of years, reading as we know it begins around 3200 BCE with the development of logographic writing, and evolves significantly with the later advent of syllabic and alphabetic systems.