Despite this, net use among this group is growing with the numbers trying the net doubling since 2011, it found.
It also revealed that 25% of disabled people have yet to use the net - a figure campaigners called "shocking".
About 88% of all UK adults, about 46 million people, used the net in the last three months, it said.
The ONS bulletin on net use in the first three months of 2016 revealed that 91% of people living in London went online regularly. By contrast, in Northern Ireland only 82% of people are regular users of online sites and services.
Pete Lee, from the ONS's surveys and economic division, said the statistics revealed some significant changes in usage and exposed those sectors of the population where net use remained low or patchy.
Net use among women aged 75 and over had grown by 169% since 2011, it found. However, it found that a significant number of older people who start using the net did not do so consistently. About 5% of those aged 75 or more had stopped using the net in early 2016.
"While we have seen a notable increase in internet usage across all groups in recent years, many older and disabled people are still not online," said Mr Lee.
Mark Atkinson, chief executive at disability charity Scope, said it was "shocking" that 25% of disabled people had not used the net as it created a significant digital divide.
"Digital access has the power to make life cost less," he said. "Many people go online to compare the best consumer deals and offers."
"Life costs more if you are disabled, from higher energy bills to specialist equipment. Scope research shows these costs add up to on average Â£550 per month," he added.
Almost two-thirds of people aged over 75 have never gone online, suggest figures from the Office for National Statistics (ONS).