Nearly one million people aged over 75 live in poverty and need more help from the government, a report suggests.
It also suggests the income of those aged over 75 is, on average, £3,000 a year less than younger pensioners the BBC reports.
Pensions minister Baroness Ros Altmann said the government had run campaigns to encourage pension credit claims.
In its report, Independent Age defines the “poverty threshold” for a single pensioner as £182 a week before housing costs. For couples, its figure is £272 per week.
Independent Age’s chief executive Janet Morrison said the UK’s 11.8m pensioners could not be considered as “one group”.
She added: “It would be foolish to assume that inequality simply ceases to exist at retirement age, but that is exactly what some of the recent rhetoric around ‘intergenerational unfairness’ does.
“The ‘silent generation’ of older pensioners, renters and single women have missed out on many of the gains of recent years.”
These, the charity says, include pension credit, a benefit that tops up low income.
An estimated 750,000 over-75s are entitled to pension credit but are failing to claim.
There are 11.8 million pensioners living in private households in the UK, of whom 4.8 million are aged 75 and over, it adds.
According to the charity’s report, an estimated 950,000 pensioners in this older age bracket live in poverty.
The report is based on analysis of the Family Resource Survey for the financial year 2013-14, published by the government and National Statistics.
It includes private households, but not those living in non-residential settings such as care or nursing homes.
Baroness Altmann said the government wanted pensioners to have the money they are entitled to.
“I am concerned if older people feel uncomfortable about claiming and we have tried to make the process easier. We’re committed to protecting people’s incomes in later life,” she said.
She also said the government had increased the value of the state pension and pension credit “significantly in recent years” along with offering such benefits as free insulation for homes and protecting winter fuel and cold weather payments for pensioners.
BBC social affairs correspondent, Michael Buchanan, says welfare cuts since 2010 have targeted people of working age while protecting the income of older people.
For example, the state pension has been subjected to a triple lock since 2010, ensuring it rises each year by the higher of inflation, average earnings or a minimum of 2.5%.
Independent Age is also calling for pension credit – which unlike the state pension, increases only in line with average earnings – to be subject to its own triple lock.