The state pension age would need to rise to 72 in 20 years' time to keep the cost to the government at the same level as in 1981.
That is the conclusion of the Pensions Policy Institute think tank in its submission to the government consultation about the issue.
It adds that people would need at least 10 years' notice of a policy change in order to adjust their retirement plans.
The pension age will rise, but there is debate over the speed of change.
The state pension age for women is already being raised to 65 by 2020.
Now the coalition government wants to raise the age at which people are eligible to draw the state pension faster than the previous government planned to do.
The Labour government was planning to raise the state pension age to 66, starting in 2024, with the pension age eventually reaching 68.
But the new government is holding a consultation to on whether to make the change earlier. It will not be sooner than 2016 for men and 2020 for women.
The Pensions Policy Institute's submission explores how much it will cost to maintain a roughly constant proportion of an individual's total adult life in receipt of the state pension.
This takes into account the expected improvement to life expectancy.
In concludes that:
- To stay at the level seen in 2010 - the state pension age would need to go up by just six months to 66.5 by 2030.
- To keep it at levels seen in 2000 - the state pension age must increase to 68 by 2030
- To return to levels seen in 1981 - the pension age would need to go up to 72.
However, people would need plenty of notice if this was to happen, the think-tank said.
Men would need at least five years' notice, and ideally more than 10. Women would need more than 10 years' notice of a policy change.
"This is because women tend to drop out of the labour market at a younger age than men," it said.