The study suggests that the average Briton loses one friend every ten months as they get older. The distinctive circle of friends falls from around 34 during late teenage years to a meagre 13 intimates by the 40th birthday.
As people mature, move away, have families or just drift apart they lose friendships at the rate of 1.2 a year, said the research by telecoms firm Dot Mobile.
The study points out that the average number of good friends to visit is 24, a figure which is more when younger and less when older. For example, students have about 35 friends, but an average parent considers just 16 people to be his or her friends.
The study also says that the cost of keeping in touch with this close circle of mates is high. Together, close friends cost the country 41.5 billion pounds a year, said Dot Mobile, which surveyed 1,000 adults aged 18 to 60 years old.
The survey shows that making and keeping friendships costs the individual around 55,000 pounds over a lifetime or, broken down, around 37.20 pounds per pal per year considering socialising, birthday presents, travel and phone bills.
British adults have an average of 24 friends on whom they spend more or less 890 pounds collectively a year.
The survey also found that people over-forties have fewer friends, but spend more, around 62 pounds per year on each friend.
The research also said that some people are choosier than others about who they regard as a friend and who they only feel is a colleague or an acquaintance or just someone they know socially through others, for example.
And teenagers have large groups of friends while, as they get older, they go for quality not quantity, said the research.
"Young Brits are a sociable bunch so it makes sense we're at our most popular before we're 25. Students are always meeting new people during their time at college or university, so it's no surprise they have more friends than any other group included in this research," the Scotsman quoted Naomi Jeanes, marketing manager for Dot Mobile, as saying.
"As our research shows, friendship can be an expensive business, too, and, ironically, students spend the most on it. However, the rate at which we shake off our friends between 25-40 is quite shocking, but, for us Brits, it's clearly about quality, not quantity," she added.