Passengers are travelling in carriages which were typically built in the mid-1990s, Office of Rail and Road (ORR) statistics show.
Press Association analysis found the average age of 21.1 years is older than at any point in publicly available records and 60% older than in 2006.
The ORR has previously said older trains can result in worse reliability, less comfortable journeys and poorer performance than modern versions, although it notes that older rolling stock can be refurbished.
Travellers using the Caledonian Sleeper service between London and Scotland have to put up with Britain's oldest trains at 42 years old.
Merseyrail, which runs trains in Merseyside, has the second oldest fleet at 38 years old.
Both operators plan to introduce new rolling stock in the coming years.
TransPennine Express, which operates in northern England and Scotland, has the newest trains at an average of just nine years old.
Campaign for Better Transport chief executive Stephen Joseph claimed the age of Britain's trains shows "just how far the railways have to go to modernise".
He said: "We've been promised new trains by several train operators and some are under construction - we now want to see these promises turn into reality.
"While some, like the famed Pacers in the North of England, do deserve the scrapyard, others can be refurbished to modern standards and could help deal with overcrowding on parts of the rail network."
Pacer trains were built in the 1980s using parts from buses and were only intended as a short-term solution to rolling stock shortages.
ORR data shows the average age of rolling stock between January and March each year since 2001.
A number of new trains were introduced following the end of British Rail in the mid-1990s but the average age has risen during the past decade.
Bruce Williamson, spokesman for pressure group Railfuture, said this highlights "an uneven feast or famine in the railways when it comes to investing".
He added: "We've still got a lot of catching up to do."
The Rail Delivery Group (RDG), representing train operators, says more than 5,500 new carriages will be in use across Britain by the end of 2020 and many other trains are undergoing multi-million pound refurbishments.
Chief executive Paul Plummer said: "This will help to deliver our commitment to boost customer satisfaction so that Britain continues to have the most highly rated major railway in Europe."
But the launch of a new fleet of trains costing £5.7 billion in October was marred by water pouring out of a faulty air conditioning unit and a 41-minute delay.
The problems hit the first passenger service by the new Hitachi-built Intercity Express trains as it travelled from Bristol to London.
Trains in London and south-east England are typically 18 years old, while regional services are 24 years old.
The latest research by watchdog Transport Focus found that fewer than seven out of 10 (68%) passengers on regional trains are satisfied with the upkeep and repair of carriages, down by two percentage points in the last 12 months.
Manuel Cortes, general secretary of the Transport Salaried Staffs Association, said the rolling stock figures "come as no surprise to those of us who use trains on a regular basis".
He went on: "Passengers will continue to get wet on leaking carriages and overheated on sunny days.
"Britain used to be proud of its reputation for giving the railways to the world. Now we just have, for the most part, a clapped-out system."
ORR figures show that £4.2 billion of taxpayers' cash went to the rail industry in 2016/17.
Taking inflation into account, this is down almost 13% on the previous year but more than twice as much as British Rail used to get before privatisation.
Private investment in rail reached a record £925 million in 2016/17, according to the RDG.
Rail fares go up by an average of 3.4% across Britain on January 2, the largest increase in five years.