“Formal kindergarten education”
Studies have compared groups of children in New Zealand who started formal literacy lessons at ages 5 and 7. Their results show that the early introduction of formal learning approaches to literacy does not improve children’s reading development, and may be damaging. By the age of 11 there was no difference in reading ability level between the two groups, but the children who started at 5 developed less positive attitudes to reading, and showed poorer text comprehension than those children who had started later.
There are several strands of evidence which all point towards the importance of play in young children’s development, and the value of an extended period of playful learning before the start of formal schooling.
In my own area of experimental and developmental psychology, studies have also consistently demonstrated the superior learning and motivation arising from playful, as opposed to instructional, approaches to learning in children.
This does not address the question regarding numbers, but states that the early introduction of formal learning (for literacy) may be damaging. It is reasonable to suggest that this may also apply to formal learning of numbers.