Image copyright Caroline Montague/Thomas Tozer Image caption Caroline Montague in her 20s and her son Thomas Tozer, 25

They are often said to be the best years of your life, but how different is it to be in your 20s now, than 10 or 20 years ago?

Thomas Tozer is 25 and lives in London. His twin brother also lives in the city. They meet up every week but he would not consider him one of his closest friends.

And Thomas is not alone.

A new index which looks at people's wellbeing has found that 20 to 29-year-olds today are feeling more distant from their family and close friends than 20-somethings were in the previous two decades.

They are also less likely to feel a sense of belonging in their community and wider society.

Originally from Devon, Thomas says his closest friends now are ones he's met in the last five years.

He lives with one of them and another he's in touch with every day via Whatsapp.

"Unless social media is backed up by seeing each other, it kind of peters out and starts to feel a bit hollow," the York graduate said.

Young adults in 2015 were 80% less likely to say one of their three closest friends was a relative than those in 1995. This was used by the index to measure the quality of family relationships.

"If I had a problem, or was upset, I wouldn't go to my parents, I'd speak to my friends," Thomas said.

Thomas's mother, Caroline Montague, has a different experience.

"My father was definitely my best friend. He was always there," the 64-year-old said, thinking back to her 20s.

Caroline spent her 20s living in Bristol, before moving to Bath. At 20, she had got a job as a trainee travel clerk and lived above the travel agents - about two miles from her father's house.

"My life was basically work," she said. "I had a mortgage but worked seven days a week to pay for it - with a bar job at the weekend. I had a lot of responsibility from the word go. I got myself from the bottom to management."

Place and family were crucial to her sense of belonging at that time.

"I loved where I lived and definitely felt I belonged. For me, I never properly left home until I was in my 30s," she said.

For Thomas, belonging comes from his practice of Buddhism.

"It's an important part of my life and identity. It defines a lot of who I am," he said. "Without that I wouldn't feel explicitly connected to London or home (Devon)."

So-called "belonging wellbeing" declined by 32% between 2005 and 2015.

The index puts this down to falls in volunteering, being part of a religion and an interest in politics, which the foundation says are activities associated with a sense of belonging.

How do you measure wellbeing?

The Intergenerational Foundation - an independent think tank focused on fairness between generations - has used data from large panel surveys to create an Index of Wellbeing.

Five different wellbeing areas were taken into account - relationships, economics, health, personal environment and belonging - at three different snap-shots in time: 1995, 2005 and 2015 for people aged 20 to 29 at each of these points.

More than 1,500 people in each wave were then scored on their wellbeing.

The index also found close friendships declined between the 2005 and 2015 cohorts of 20-somethings - falling 6% in 10 years, despite rises in social media in the same period.

This was measured by asking how often people are in touch with their three closest friends.

Angus Hanton, co-founder of the Intergenerational Foundation, said the cracks appearing in this generation's closest relationships and sense of belonging should be of concern to everyone.

But the index doesn't tell everyone's story.

Image copyright Anne Hayman Image caption The Hayman family now and Anne Hayman in her 20s at home in Newport

Will Hayman would count his older brother among his closest friends.

"That's something that's changed as we've got older," the 27-year-old from Newport, south Wales, said.

The pair speak almost every day and he's in touch with his other friends most days through text or Whatsapp.

These friendships are "really important to everyday life", the secondary school teacher said.

"Most of them are old friends from childhood. I would consider them almost family friends. That feeling that you go back so long with the same interests and backgrounds."

'Finding out who you really are'

Will has recently moved from nearby Cardiff back in with his parents in Newport to save money.

"Moving back home means I can do more than just pay rent. I was lucky they were just down the road," he said.

Bucking the trend for his age group, Will said he does feel like he belongs.

He doesn't participate in a religion or volunteer, but he knows and socialises with lots of likeminded people in his community and plays tennis in a local club.

Beyond Newport, he said he feels part of UK society: "I'm engaged with politics, I have lots of friends in London and spent five years living in Liverpool."

"My 20s so far have been about putting my interests, hobbies and socialising first and finding out about who you really are," he said.

Image copyright Anne Hayman Image caption Will Hayman, 27, and his mother Anne - seen here in her 20s

By contrast, Will's mother, Anne Hayman, had had her first child, bought a house and was preparing to get married by his age.

Having left school at 16, Anne lived in her family home in Newport until she was 26 and worked in the town centre's reference library.

"I had a big circle of friends - we'd be out all the time at gigs, pubs or the cinema," Anne, now 57, said.

"I was always skint but I had enough to go out and go on holidays."

The security of family and close friends instilled a sense of belonging, with Anne adding that her sister was definitely counted as one of her closest friends.

"I was always politically aware and part of the Young Socialists in my 20s. When I joined the trade union, I would go to Labour party meetings - that was a community to which I belonged," she said.

"I think we had a fantastic 20s."

Elsewhere, the index found a decline in health wellbeing of 11% between the 1995 and 2015 cohorts, which points to a deterioration in how 20-somethings assess their own physical health.

Report author David Kingman said this "could be due to an increase in obesity or it could be due to greater exposure to social media and anxiety over body image since younger generations are drinking less, taking drugs less, and exercising more than previous generations did at the same age".