While Michael Douglas, 72, and Catherine Zeta-Jones – who have a 25-year age difference – did separate briefly in 2013, they are now said to be happier than ever. Picture: AFP

London - They often face raised eyebrows, sniggers and even cruel whispers that their marriage will never last. But couples with a large age difference between them should perhaps pay some heed.

For marriages where one partner is a lot younger than the other are likely to hit trouble quicker, researchers say.

Cougars married to toyboys and men with much younger wives see the steepest decline in their relationship six to ten years after tying the knot.

This can be because the younger partner is more eager to have children or the older partner retires. Or there may be other sudden life changes, such as financial pressures, that accentuate the generation gap.

Some celebrities have shown that age difference is no barrier to lasting happiness. Joan Collins, 84, married Percy Gibson, 52, 15 years ago. Showbusiness legend has it that Miss Collins, reminded of the 32-year age gap, said nonchalantly: "If he dies, he dies."

And while Michael Douglas, 72, and Catherine Zeta-Jones – who have a 25-year age difference – did separate briefly in 2013, they are now said to be happier than ever after nearly 17 years of marriage.

The US study, which examined data over 13 years from nearly 20 000 Australians, found that men who were married to younger women were the most satisfied.

Those whose wife was seven or more years older were the least satisfied. Women were happiest when married to a younger man, despite previous studies suggesting they pick older husbands who are likely to have more money.

But after six to ten years of marriage, the happiness of age-gap couples fell steeply, cancelling out the extra happiness of marrying someone younger and making them even less happy than those with spouses of a similar age. Lead author Professor Terra McKinnish, of the University of Colorado, said: "Couples have to make a lot of joint decisions, such as where to live, how many children to have, and how to spend their money.

"Couples who are more dissimilar, and the age difference is one source of dissimilarity, may be less likely to agree on these joint decisions. This may become an increasing source of conflict."

The study, published in the Journal of Population Economics, says: "The fact that the husband and wife are in different points of their life cycle may have different effects at different points, such as when one partner wants to have children or when one wants to retire."

Ammanda Major, of Relate, said: "We know that age gap relationships absolutely can work, but we also know that there are some common challenges. When the relationship matures, people may begin to look at their partner in a different light or their priorities may change, and this is when problems can occur."