Based on my experience as a learner and a teacher, plus tons of reading, the first obvious answer is, it depends. The following are generalizations, and apply to the mythical average child. Also bear in mind that most of the users of the SDMB are probably at least somewhat above average, and probably so are their families and others they associate with. What you as an individual perceive as “normal” is probably above average in the general population.
All language skills are best learned prior to puberty, the earlier the better. Preschool and early elementary school is the absolute best time to learn a second (or third or fourth) language. Mathematics beyond the basics are best learned later, in the immediate pre-pubescent years. Skills requiring small muscle coordination are very difficult for children below the age of about 5 or so, more so for boys than for girls. (If you were to look at an x-ray of a little boy’s hand and a little girl’s hand, the girl’s would be more developed.) Hand-eye coordination and large muscle skills need to be developed from an early age. Like language, they are much more easily acquired when young, and become progressively more difficult to learn after puberty and beyond. All of the above is based on the normal physical and neurological development of humans, regardless of culture.
Bottom line? Our educational system is in large degree upside down. Five-year-olds should be learning French and Chinese and any other languages they can be immersed in. They should be learning to work with other people in small groups and then in larger groups. They need to develop the skills they will need in later learning, such as paying attention, listening, courtesy, and so on. Young children should spend a lot of time engaging in activities that will help them develop coordination. They should not be imprisoned behind a little desk trying to make letter shapes.
Instead, what do we do? We expect the little children to learn arithmetic, the concept of which the average brain at that age is not really ready to grasp. Then, once they reach the teen years, just when the language-learning portions of the brain have mostly atrophied, we expect them to learn a foreign language.
That said, lots of children who are above average and are in an enriched environment will learn to read well before the age of 6, the usual age for first grade, as both my daughters and I did. My younger daughter did addition and multiplication in her head *for fun * before she ever set foot in a kindergarten.
Again, these are all very broad generalizations, and there are tons of exceptions. It is my firm conviction that there is no point at all in trying to teach many of the alleged “pre-reading skills” to a child whose brain has not yet developed to the point of being able to process the very artificial concept of marks on paper = words. Just like walking and talking, they will learn only when ready, given the means and opportunity.