Each Child Is Unique : Although every child goes through similar stages of development, the process may vary from one set of children to another, and also from one child to another. This difference may be related to the socio-economic and cultural background that children come from, apart from individual differences like those in intelligence, attitudes, etc. For example, it is common to find a street vendor's six or seven-year-old calculating the cost of three or four items in a very short time.
For another example, girls in rural households are expected to cook from a very early age on. Consequently they have an intuitive grasp of proportionality, conservation of mass, and many other mathematical concepts. This wouldn't be true of a child of the same age from an urban middle-class background, or even a boy of the same age from a rural background.
Apart from social, cultural and economic factors, there are psychological aspects that vary from child to child. This is often reflected in the different strategies that a child evolves for grasping new concepts. For example, a child may either use a 'count all' or 'count on' strategy to begin with, or proceed to more efficient ones as she grows. Some children regroup numbers in fives, while others may do so in threes, or even tens.