The UK's population of over-80s is expected to double to five million by 2031, according to experts.
Older workforce: Changes expected in years to come
Calculations for the government predict the number of pensioners will rise while the number of children will fall.
But due to expected changes in pensions, the working population is expected to age over the same period.
Overall the population may reach more than 60 million by the middle of the century - though it may then start falling back again.
According to the Office of National Statistics projections, produced by government actuaries and drawing on data from the 2001 census, the UK's population will continue to age as birth rates fall and life expectancy increases.
Experts say there may be 4.9 million over-80s by 2031 and as many as seven million by 2050.
In total, there are expected to be 12.2 million people of pensionable age by 2011.
Population getting older
12m Pensioners by 2011
5m over-80s by 2031
Average Briton age to reach 43
Number of children dropping
Working age increasing
Source: Office for National Statistics
But from 2021 onwards the number of pensioners is expected to increase more rapidly to as many as 17 million people in 2060.
While the average age of Britons in 2002 was 39.3 years, by 2031 it will be 43.6 years.
Meanwhile, the number of children is expected to fall from almost 12 million today to below 11 million in 2014.
Experts are unsure of what may happen after that, but suggest the number of children may rise slowly until the late 2020s.
The government's statisticians also say average working age will continue to change. In 2002 the number of people of working age was 36.6 million.
That is expected to rise by just over a million by 2011. The number of workers will increase yet again by 2021 as rises in the state pension age for women come into effect.
Projections also suggest the working population will continue to age. While workers aged 30 to 44 years old will fall, the 45-59 age group is expected to increase over the next 15 years.
The UK's population is projected to increase gradually from the current 59 million to reach 64.8 in 2031, an annual growth rate of 0.3%.
Experts think the population may peak at 65 million people in 2050 but then start to fall again.
However more revisions and predictions may be made be made in the coming year to take into account recent controversial corrections to census results.
Population predictions are based on birth rates levelling at 1.74 children per woman and immigration continuing at the current rate of 130,000 people a year - although the report's authors say migration remains largely unpredictable year-to-year.
The populations in England, Wales and Northern Ireland is predicted to continue rising for the time being. Scotland's population is expected to continue to decline.