The number of people aged 80 and over in the UK is expected to more than double to six million by the middle of 2037, according to figures published by the Office for National Statistics.
It includes a more than eightfold increase in the number of centenarians.
The average age is projected to rise from 39.7 years in 2012 to 40.6 years in mid 2022 and 42.8 by mid 2037.
The UK population is projected to rise by 9.6 million from 63.7 million in the middle of 2012 to 70 million by 2027.
It is expected to rise to 73.3 million by the middle of 2037, the ONS said.
The ONS said a breakdown of the projected 9.6 million increase by 2037 was expected to be made up of 5.4 million more births than deaths (57% of the rise) and 4.2 million in net migration into the country (43%).
The increase in the numbers of older people means that by mid-2037 one in 12 of the population is projected to be aged 80 and over.
The number of people aged 90 and over is projected to more than triple, while the number of centenarians is expected to rise from 13,000 in mid-2012 to 111,000 in mid-2037.
Meanwhile the number of people of working age for every person of state pension age is projected to fall from 3.2 in the middle of 2012 to 2.7 in 2037, taking into account planned changes to eligibility for the state pension.
The ONS said the average completed family size is projected to be 1.89 children by 2037.
The number of children aged under five is not projected to change much over in the next 25 years.
But the number of primary school-age children is projected to increase by 13% to reach 5.7 million. Over the same period, the number of children aged 12 to 16 is projected to rise by 10% to 4.1 million.
The projected rise in the UK population is greater than the current 8.3 million population of London.
Richard Pereira, head of population statistics for the ONS, said: "These population projections are used across government in terms of setting policy, they are used by the Office for Budget Responsibility (OBR) as a key input for their long-term fiscal projections, they are used by the department for work and pensions for policy on benefits and pensions, and they are used by people like the Department for Education."