By Flora Botsford BBC Madrid Correspondent

Many countries are facing the problems of an ageing population, as people around the world get older and families are having fewer children.

Not only in Europe and the United States but in large parts of the developing world, societies are ageing while governments stretch their social services to cope.

I'm not moving from this house, that's for sure

Rosa Maria Eguren Zubigaray, 83

By 2050, Spain will have the highest average age in the world.

As representatives of more than 160 member states gathered in the Spanish capital, Madrid, to attend the Second United Nations World Assembly on Ageing, I explored the way old people live in a country famed for its family traditions and laid-back lifestyle.

The vast majority - 85% - of pensioners in Spain live with family or in their own home, a high proportion by European standards.

But residential care for the elderly is growing in popularity.

Family option

Surrounded by her 12 grown-up children, 26 grandchildren and two great-grandchildren, I met Rosa Maria Eguren Zubigaray, an 83-year-old widow, and now head of the family.

She still lives in the family home, a spacious flat in central Madrid where at one time the children were stacked up in bunk beds, six to a room.

By 2050 Spain will have the highest average age in the world

Her one unmarried son and two unmarried daughters live with her and help in their mother's daily care.

This is very much the traditional life for Spain's elderly population, and one Rosa Maria would not change for the world.

"There are more and more old people's homes", she says, "and fewer people who are prepared to make the sacrifice, or resign themselves to this situation - it's true.

"But I'm not moving from this house, that's for sure."

But moving away from home into state-run, semi-private or private residential care is the reality for a growing number of pensioners in Spain.

Residential care

I visited a vast home for the elderly in Colmenar Viejo, just outside the Spanish capital.

Offering more than 600 people a permanent home, Gonzales el Bueno is the biggest residential care centre in Europe.

Nowadays our children all have to look after their own families, so they don't have time to stay with the old people

Fernando Monerris Terres Secretary of Colmenar Viejo Residents' Association

However, with more people living longer, the problem is dealing with the demand: 20,000 elderly residents are on the waiting list in the Madrid area alone.

The authorities currently foot 60% of the bill for public residential care; the remainder the residents pay for it out of their pension.

So why did some of the residents choose to come here?

"We're more comfortable here than in our own homes" says Dolores Atienza Medina.

"We have heating and hot water, we have meals at regular hours, you don't have the bother of preparing your own, here it's more comfortable than at home."

And Fernando Monerris Terres, secretary of the Residents' Association at the home explains:

"Nowadays our children all have to make their own homes, look after their own families, their own children, so they don't have time to stay with the old people."

But Spain and other countries in the industrialised world will have to decide whether state provision for the elderly is affordable in future.

Funding old age

If the proportion of people over 65 keeps growing, and the birthrate keeps slowing, the individual will surely have to pay.

If the demand for residential care continues to grow, what may happen in future is that even those at the bottom end of the scale may have to increase their personal contribution.

Many old people prefer to stay with their families

Spain could even be looking at Tony Blair's idea of forcing people to sell their private property in order to pay for their own care in old age, according to Eduardo Seyller Garcia, the manager at the home.

"A person's individual contribution, whether it be wealth in property, in shares, or pensions; in future they will have to bring it with them to pay for their own care" he says.

For the majority of Spain's elderly population, these calculations remain hypothetical: something for the next generation to worry about.

With adequate state pensions and family support, the emphasis in Spain is on enjoying old age to the full, with living longer being something to look forward to, not dread.

Around the city, you see numerous older people enjoying a life of ease: playing cards, chess, or petanca , a game like the French boules, in the sunshine; strolling with their grandchildren in the park, knitting, chatting, or just sitting.

Spain's six and a half million over 65s appear very relaxed.

Despite government predictions that the proportion of pensioners will rise from 16.2% to 20% by 2010, the debate about public funding for senior citizens has barely started.