Something odd. My niece taught herself to read this past year (when she was 5). My sister's best friend's sister runs a daycare and she could not believe that a 5 year old could read. So the friend videotaped my niece reading the newspaper.
Apparently my niece's friends who are mostly 6 can't read (I would have thought that most kids would have picked up some reading on their own).I thought it was interesting that my niece taught herself to read. But i never thought it was that special. I mean she will be starting grade 1 in 2 weeks. I was surprised that the friend's sister could not fathom a 5 year old reading. It's not like she was 2. Now if my other niece could read that would be impressive. 
I'm sure your niece is read to a lot, plus you might have been teaching her without realizing it.
The pre-school teacher told me to stop teaching Bryan how to read & I mistakenly listened, after all it was a Montessori school. He was doing so in Kindergarten (age 5) in Massachusetts. The Kindergarten teacher in Pennsylvania wouldn't let him read so he stopped. I don't think she liked him. After that experience I went ahead & taught Craig to read before he started Kindergarten in Pennsylvania knowing he would have the same teacher as Bryan. The teacher liked Craig & let him read to the class. When you do the ground work for reading, teaching is really just letting them read, with help, on their own & building on their skills.
To answer your question, formal teaching began in 1st grade around age six. I suppose that could have changed by now.
