The average age of people in England and Wales identifying as Christian is now above 50, while a majority of young adults say they have no religion, 2021 census data shows.
Nearly three in ten who identify as Christian are 65 and over - up from just over two in ten a decade ago.
An ageing population and differences in the way people choose to self-identify were among the factors driving the trend, experts said.
The median age - or exact midpoint - of people identifying as Christian was 51 at the 2021 census, up from 45 in 2011, according to new analysis from Britain's Office for National Statistics (ONS).
This is the oldest average age among all main religious groups.
The youngest average age is 27 years, for people identifying as Muslim, up from 25 in 2011.
Most of the other main groups have seen an increase, with Hindu up from 32 years to 37 years, Sikh from 32 to 37 and Buddhist from 37 to 43, while the average age of people identifying as Jewish is unchanged at 41 years.
The figures show a growing proportion of people in England and Wales who identify as Christian are aged 65 and over, up from 22.3% in 2011 to 29.0% in 2021.
At the same time, the proportion of those aged 21-25 identifying as Christian has fallen from 5.1% to 3.9%.
The average age of people saying they have no religion has increased from 30 to 32.
Only 8.8% of those who said they had no religion were aged 65 and over, compared with 18.6% of the overall population who belong in this age group.
More than half of people in every year from age 22 to age 30 said they had no religion, with the highest proportion for 27-year-olds (53.0%).
"Many factors can cause changes in the size of religious groups, including changing age structure, people relocating for work or education, and differences in the way individuals chose to self-identify (or how children's religious affiliation was reported) between censuses," the ONS said.
Humanists UK, which ran a campaign ahead of the two most recent censuses encouraging non-religious people to tick the form's "no religion" box, said the latest figures "underline the archaic place that collective worship and faith-based discrimination have in our schools".
Chief executive Andrew Copson said: "They make plain that the UK faces a non-religious future.
"This is in stark contrast to how our state institutions operate today.
"No other European country has such a religious set-up as we do in terms of law and public policy, while at the same time having such a non-religious population.
"Politicians should look at today's results and recognise they must renegotiate the place of religion or belief in today's society."