The shifting balance is part of an overall ageing of the population and will mean, for the first time, the over-60s will outnumber under-16s.
A drop in the birth rate is expected to cut the number of youngsters by about a million over the next two decades.
The fall of almost 10 per cent will mean the number of under-16s dropping from about 12 million in 1996 to 11 million by 2021.
At the same time, the number of people of pensionable age is expected to rise from 10.7 million in 1996 to 12 million in 2021 and 15.5 million in the 2030s.
The year 2008 is predicted to be when the age balance actually tips. In 2007 there will be 11.5 million children and 11.3 million pensioners. In the following year, that will change to 11.4 million children and 11.5 million pensioners.
The shift is part of an overall trend towards an older society, according to the projections published by the Office of National Statistics.
They show that the average age of people in the UK will rise over the same period from 38.4 years to 41.9.
The number of people aged over 80 will also continue to grow from 2.4 million in 1996 to 3 million in 2021 - and go on rising to a peak of 5.5 million by the middle of the next century.
It is also expected that the workforce will be older with two million fewer people aged 16 to 44 and 2.5 million more aged between 44 and 59.
The balance between the working population and non-workers - children and pensioners who are reliant on those in work - will also change.Reuse content