Families and older tenants now form a large and growing proportion of private renters, with the number of tenants over 30 increasing 6% in the past year.
Many are unable to get on the housing ladder by their mid-30s and therefore continue to rent in the private sector for longer.
The over 30s now account for nearly 60% of all tenants in England, Scotland and Wales, according to the Countrywide Monthly Lettings Index for January 2014.
The number of tenants in the 41 to 50 age group rose more than any other, up 2.2% year-on-year to form 16.2% of all tenants.
While some older people continue to rent because they can’t afford to buy, others do so because it gives them greater flexibility and job mobility, Countrywide said.
Furthermore, the number of families living in private rented accommodation has also increased in most regions, with London seeing the greatest year-on-year increase, up 6%.
The proportion of younger people renting privately has fallen 5.8% over the past 12 months. This is partly due to Help to Buy, which has helped thousands to get on or move up the housing ladder.
But many younger people are also forced to live in the family home for longer, due to rising living costs and job instability, Countrywide said.
Recently-published government figures showed that 22% of 20 to 34 year olds in London are living at the family home, up from 18% a decade ago.
The survey also showed that the UK average monthly rent in January 2014 was £859, up 0.6% across the month and 2.9% year-on-year. Over the past 12 months, average monthly rents have increased in eight out of 10 regions, with Scotland seeing the most significant increase up 11.7% to £639.
Nick Dunning, group commercial director at Countrywide plc, said: “The large increase in rents in Scotland over the past year could partly be due to the fact that tenant fees can no longer be charged in Scotland. The cost of carrying out the work by the letting agent is instead passed onto the landlord who subsequently covers their costs by charging higher rents.”