Some men held in upstairs accommodation at HMP Brixton struggled to collect their meals or make it to social activities, a watchdog report found.
They also faced difficulties accessing a mobility scooter located at ground level.
The Independent Monitoring Board for the south London prison found that its “cramped” cells cannot accommodate two men humanely, particularly if they are old or infirm.
The majority of men aged over 60 and all those over 70 were held in G-wing, where there is only one cell on the ground floor and no lift.
The report said: “This made it difficult for men to get their meals, access social activities and exercise, and use the one mobility scooter on the ground floor.”
As of August, 21 inmates were assisted by “buddies”, who collected their meals and did other tasks like making the bed.
The IMB called on HM Prison & Probation Service to end the practice of allocating men who are aged over 65, or have chronic mobility problems, to prisons with minimal or limited ground floor accommodation, and where they may have to share cells with bunk beds.
Last year, a joint assessment by two watchdogs warned that the prison service and local authorities are failing to plan for a rise in elderly, ill and frail inmates
The report by HM Inspectorate of Prisons and the Care Quality Commission found many older jails are ill-equipped for prisoners in wheelchairs or with mobility problems.
There were 13,636 prisoners aged 50 or over in England and Wales in September, representing 16% of the prison population.
Projections indicate that the number of individuals in older age brackets held in custodial settings is likely to increase.
The report on HMP Brixton found the prison has improved significantly over the past year.
Graham King, chairman of the IMB, said: “The Governor and his team, including staff at all levels and in agencies, have pushed forward with vision and commitment to make Brixton a fairer and more decent prison.”