The charity's report examines the pandemic's "appalling" impact on those aged 60 and over in January and February.
Britain then faced social distancing, loss of routines and limited access to services for pre-existing or new health issues.
Around 4.3 million - 27 percent - said they could no longer walk as far while a quarter were living in more physical pain.
Some 17 percent said they were less steady on their feet and 12 percent felt less independent.
Ten percent who had previously been able to get up and down the stairs said they were now finding it difficult.
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COMMENT BY CAROLINE ABRAHAMS
We seem to be at that stage in the pandemic now when we hardly dare hope the worst may be behind us.