When should formal learning start for kids?

Is it four, five or six? An elaborate article on BBC news discusses the age for starting formal education in primary schools in England.

"There is no evidence that an early introduction to formal learning has any benefit, the review says, but there are suggestions it can do some harm."

The situation in India is not much different, where kids start going to schools (nursery) at age of three. And it is not just the play component that is introduced to the kids at this age. Proponents of early education claim that the learning power of people deteriorate as they grow old so its better to catch them young.

Nothing has been proven for sure, in absence of which, the best strategy is to take a path midway.