Middle age, it oft is said, is when your age starts to show around your middle. And for men, it seems, the moment is marked by the inexorable rise in the position of their trouser waistband.

A survey shows that the last time most men are able to fasten their trousers around anything resembling a natural waist is at the age of 39. After that, the only way is up, or down.

"Over achievers", as they are known in the rag trade, hoist their trousers so high by the age of 57 the waistband can be just 7in (17.8cm) under the armpit. The "under achievers", making up about 20%, plump for below, fumbling to fasten belts, buttons and zips they can no longer see.

"The changing fortunes of a man's trouser waistband can often become a metaphor for his life," said Paul Baldwin, director of menswear buying for Debenhams, which commissioned the survey to chart the force of gravitational pull beneath the male midriff.

Boys wear their trousers around their waist, the thinnest part of the body between the rib cage and hips, until the age of 12, because their parents buy their clothing for them, concluded the survey of 1,000 males. But waistbands plunge with the advent of teenage hormones, plummeting to 5in towards the apex of the hips, and far below the underpants position by the age of 16.

Dressing for work sees a gradual upward creep between 16 and 20 years.

By 27, the waistband starts returning to the natural waist, a position largely maintained until the age of 36, and influenced by factors such as career progression, marriage and impressing prospective in-laws. The critical turning point is 39 and the demise of the washboard stomach. And by 45 trousers will be worn at least 2in above the waist, rising to 5in by the age of 57. The frailties of old age bring some benefits. With 65-year-olds, waistbands are 3in above the waist, and for those aged 75 just an inch.

Baldwin said the research would help designers produce better-fit trousers for men regardless of their age. "However, our hardest task continues to be persuading men to confront the fact that their trouser waistbands have risen. From our experience men still prefer to assume that their trousers no longer fit because their legs have suddenly grown."