My sons were 4 when they wanted to learn how to read from their older sister (7 at the time). I diverted their attention and learned them how to play chess instead
(anything that's not taught in primary school would do).
The reason was this: if they can read before primary school, they will certainly be bored out of their minds, and would be detrimental to their school experience.
Today, I still think it was the right thing to do.
I learned to read before primary school, I was bored out of my mind. The school offered to make me skip a class, my parents declined because they thought being too young would be detrimental to my school experience.
Knowing me, that was a mistake. To this day, I regret not being put through an accelerated curriculum, or at least being taken out of school and given a chance to spend that time learning on my own.
Overall, despite being bored in primary school, I am still glad I learned before, as it afforded me the opportunity to learn more things by reading on my own, and it protected me from the general poor quality of reading education in school.
The reason was this: if they can read before primary school, they will certainly be bored out of their minds, and would be detrimental to their school experience.
Today, I still think it was the right thing to do.