By Matthew Grant BBC News, Calcutta

Rehena (l), Ali Hussein and Ikramul suffer from the rare disease

Only seven years old, Ali Hussein's hair is already thinning.

This is the most visible of many symptoms showing - despite being a boy - he is actually middle-aged.

He suffers from progeria - an extremely rare condition causing his body to fast-forward through the ageing process.

Like his sister and brother who also have the disease - Rhena, 19, and Ikramul, 17 - he has almost no chance of living beyond his early twenties.

The illness has stunted the development of the children. It causes other problems, such as a second row of teeth in their mouths and pale, almost translucent, skin.

'Action hero'

They are also prone to catching diseases common in the elderly. Last year, their sister, Ravena, who also had progeria, died aged 16 of pneumonia.

Yet when Ali Hussein speaks it is clear he retains the enthusiasm and hope of a young boy.

"I would like to be an actor, to drive cars and aeroplanes and be an action hero," he says.

"But after that, I would like to be a doctor, because the doctors always check me and I like to check myself, so I would like to be a doctor someday."

I would like to be an actor, to drive cars and aeroplanes and be an action hero Ali Hussein

Only 48 cases of what is formally known as Hutchinson-Gilford progeria syndrome exist in the world.

What makes this family's case unique is it is the only known example where more than one family member suffers from the disease.

This has allowed researchers to make a breakthrough in their understanding of the condition.

A team led by paediatrician Dr Chandan Chattopadhyay studied the family for two years and concluded progeria was an inherited and recessive condition.

This means both parents must carry the gene. In this case, the parents are first cousins. Neither has the disease and they also have two other children - Sangita, 14, and Gulavsa, aged two - who are healthy.

The study of the progeria family may have wider benefits, as by examining the ageing process in this unusual form the scientists hope to deepen their understanding of how ordinary humans grow old.

Father's fears

The children's father, Bisul Khan, says life has been extremely hard for him and his wife, Rajia.

The Khan family is being looked after by a charity in Calcutta

They are from a village in Bihar in India. The locals called the children aliens and as a result they grew up completely isolated from other people.

"When we were there in Bihar, every night we sat in our room unable to sleep, because sometimes one child is suffering from one thing and another child is suffering from something else," Mr Khan says.

"We used to think, my wife and I as we sat there together, how will we live our life in future? Sometimes we decided we would kill ourselves all together.

"But now the children are surviving. They are getting energy, they are happy now and they are living their lives as normally as possible."

Care from charity

During the past two years, the Khan family has been looked after by Sekhar Chattopadhyay, who runs the SB Devi Charity Home in Calcutta.

They now live in the city, although their address is secret.

The charity has helped the father find work as a security guard, but his pay is low and they are looking at other ways of raising funds for the family.

The normal human contact the charity offers the children is as important as money.

"We gave them support and we made friends," says Mr Chattopadhyay, who jiggles Ali Hussein on his knee. "Slowly I got to know the family well and how much they love me now you can't imagine."

Thanks to this support, the family say they are enjoying life much more than they ever have before.

They smile as they talk about their interests and hobbies.

Rehena says she loves Bollywood films, particularly the passionate love songs.

When she is asked to sing herself, she says she is too shy, yet it is clear she wants to and after some encouragement she does.

"I like to love you and whenever I don't see you I long to see you immediately," she sings in Hindi.