I came across the article “Increased academic rigor in kindergarten questioned” in the “Early Childhood Learning, Child Care, and CCR&R Professionals and Forum” in LinkedIn. Its US centric, but raised some interesting points. Fundamentally it says that even 5 and 6 year olds can be taught to write their first sentences, and it does seem to set them ahead for later years.
Some expressed the opinion that play was better as it was more creative than writing (which I guess is a more technical skill) – but it does indicate that teaching your children while very young (when kept in balance overall with other activities) is definitely worthwhile academically.
When I think about it, my 3 year old boy can recognize around 50 names of engines from the Thomas the tank engine series, so the idea of writing simple sentences in by 5 or 6 feels quite realistic. There certainly do seem to be different camps of people with some saying “start them as early as possible” and others saying “it does not matter – in a few years they will all be at the same level anyway”. From what I have seen however, the former camp seems to have more evidence to back it up.