I didn't learn to read until I was in first grade. I wasn't even in the first group of kids in my class to start, as somehow I didn't test quite as high on my "reading readiness" tests. But once I did start, and I figured it out, I zoomed ahead of them and was reading at a third grade level by the end of the year. Back in those days, kindergarten was a half day thing and was all about play and socialization. But never mind that; most of us had grown up in families of two, three, four kids and neighborhoods where we had lots of other kids to play with. I was lucky enough to have had parents who read to us. However, teachers tended to frown on kids who already knew how to read when they entered first grade, as it made their job so much harder.
My kids, raised on Sesame Street and The Electric Company, read to in utero and all their lives and sent to preschool, were still vastly different. Son #1 is on the ASD spectrum and went to a therapeutic preschool from 2 1/2 to 3 1/2, then to a regular one from 4-5, then started school in a special ed class for one year. He'd been fascinated by commercials all his life, and at 19 months had yelled "Tide!" and pointed at the box of detergent on a shelf in the grocery store, amazing me. One day in his special ed class, he walked into school, picked up a book, and started reading it aloud to his special friend, a non-verbal autistic girl. His teacher said she just stood there with her mouth open. None of us had any idea he could read at all. He went to a regular first grade the next year, not a happy placement as it turned out. Too many stimuli made it hard for him to learn.
Son #2 and daughter went to preschool and kindergarten at our church. This preschool had been founded prior to compulsory kindergarten in Tennessee and was much better than any of the public kindergartens in the area. It was a combination of play and socialization for the younger age groups and some academics for the older ones. Many if not most of the kindergarteners came out of there reading. Son #2 was reading a little and knew all of his letters and numbers. He was actually better with numbers and continued to be so throughout his life. When he went to public school first grade he was found to be dyslexic as well as gifted, but thanks to an excellent special ed teacher the dyslexia was worked on and he became a good reader. Too bad nobody caught his dysgraphia.
Daughter, OTOH, taught herself to read at 41/2. I think it came about because of a game we played at the supper table sometimes, which was "Think of a word that starts with the __ sound." Everyone was free to say as many words starting with that sound as they could think of, but they had to be real words. She could join in, even though she couldn't yet read, and boy, was she good at it! She'd been an early talker (probably to be heard over the noise of her brothers!) and loved being read to. One day I heard her in the living room talking to herself and peeked in to see what she was doing. She had her magnetic board and letters and was saying "Guh guh guh oh oh oh. Guh oh. Guh oh. Guh oh. GO! GO! Mommy! I can READ!" I had never seen her so excited. I showed her how to put the letters together for "dog" and "mom" and "dad" and you have never seen a happier kid. She already knew how to write her first name, so the next day I took her to the library and she got her very own library card, the prerequisite for that at our library being that the child could write their (first) name.
I don't think it matters so much when, as HOW. Talk to your child and read to your child and give them the experience of words, and they will learn when they learn.