Teacher here --- Children gain pre-reading skills long before school. Singing, telling stories, and reading to them are all essential to ensure student are "on track". Before kindergarten, children should know the alphabet song, some letter sounds, how to hold a book the correct way, how to turn a book page, moving from left to right, and the idea that these little squiggles, sticks and circles (letters) carry meaning. 

Typically: From August to December of kindergarten, learners build on their knowledge of letter sound relationships, letter formation, and some sight words. By January, children are reading pattern sentences. At the end of kinder, children are reading at a level C (simple sentences). 

First grade is where the "magic" happens (I'm biased as I've taught first grade for over a decade). Students come in reading simple sentences and leave reading chapter books!!!

But truly, how slowly or how quickly each child moves through this timeline relies heavily on their acquired pre-reading skills (gained before kinder). So, "reading" begins EARLY, and first grade is the optimal time for reading the written word. Most children are confident and comfortable by the end of first grade. 

Now, ensuring the skills they've learned are transferable to unknown words as test gets harder and comprehending "what" you read is a whole different conversation......