Not sure if there is an optimal age each child is different. With all four of my children I started reading to them while I was carrying them. I had purchased colorful flash cards with primary colors, numbers, ABC's with pictures alphabet. Pack up in bag to take with to hospital and after delivery I sat there and showed the cards and said each one over and over to them. I had packed a child dictionary too. then while in my room I'd sat up in bed with baby laying in front a put book in front of both and read each picture and its meaning.
All my children were reading before they could walk. My youngest I taught him French..well we both learned it. I had not a clue how to speak it. Bought a child foreign language program and taught both of us.
He (StrongChance, is his name) was referred for a excel school for Kngrd when tested he spoke in French, the tester had brought him back saying he didn't know his ABC's, which I knew he did. I had him tell me them and told the testing agent that it was French he was speaking. You have to get him someone that speaks it, they did and he passed with Flying French colors so to speak. The lady ask if anyone in family spoke French I told no, and told her what I did in teaching him. She said she has never heard a child speak French that good and has no one to speak it with.
All kids reading levels were above the norm when they were in pre-school. By the time they got into grade school, they were read at Middle school level. My oldest she is the bookworm of all of them she read everything, including the dictionary (Websters and the huge one that school rooms have in them).
Some say that was to young, but I didn't think so and still don't. The library was their favorite place to go. They'd get there books, sit and read them, talked about other places. Which lead them to use their imagination, initiative in seeking out answers on their own (cause mommy can't always do it for them), play alone, sit quietly, attentive and a great hunger to learn more
Schools today teach pre-schoolers on computers: math, reading etc. Where the live now, the kids have to start learning Spanish in pre-school. My reasoning in teaching them so young was because I didn't want people to call them dumb, stupid and other negatives. Like I was called, because I didn't understand quickly. I'm dyslexic, it takes me longer to learn