Streetwork found rough sleepers in Scotland lived for eight years less than those south of the Border, where similar surveys have indicated a life-expectancy of 47.
The findings sparked calls yesterday for the Scottish Government to do more to ease the plight of people living rough.
Unveiling the survey, Streetwork chief executive Claire Gibson said: "Thirty-nine years old - for most, it is considered the prime of their life.
"For us, on average, it is the age at which someone rough sleeping will die. Streetwork aims to be there to stop that happening."
She added: "In these times of austerity measures we see despair, desperation and destitution on the streets. Welfare reform, unemployment and the chronic shortage of housing are impacting on the routes off the street."
The Edinburgh-based charity's findings were backed by Robert Aldridge, chief executive of the charity Homeless Action Scotland.
He said: "In Scotland we have seen a welcome reduction in overall homelessness recently but we haven't yet put an end to rough sleeping. Our New Year resolution must be to focus on helping people off the streets."
The average life expectancy in Scotland is 76 for a male born today and 80 for a female.
Figures for the number of rough sleepers are imprecise, according to housing charity Shelter, as there are no official counts.
Last year, 145 people per month said they had slept rough on the night before applying to their council for help.
The number was down from 450 a decade ago, but Shelter insist it does not tell the whole story, as many rough sleepers do not seek local authority support.
Rough sleepers account for a tiny fraction of Scotland's homeless.
Last year 39,827 households declared themselves homeless to their council, with 80% of applications accepted. The number of applications has fallen from a high of more than 60,000 in 2005.
Scottish Labour MSP Jackie Baillie said: "For too many, there simply isn't the emergency accommodation available to ensure that they have a safe place to sleep. The SNP government needs to act - and act now.
"There are too many rough sleepers in Scotland. There are too many rough sleepers dying at an age which is far, far too young. They need support and stable accommodation to help turn their lives around."
A Scottish Government spokesman said: "The Scottish Government works closely with local authorities and their partners to prevent homelessness and rough sleeping wherever possible, and some of the projects funded by the recent announcement of £2.1 million for projects in the voluntary sector have this as their aim.
"Charities like Streetwork provide a valuable service in highlighting the plight of homeless people in Scotland, and their findings serve as a reminder of the hardships faced by those who are sleeping rough.
"Anyone who finds themselves in this position has a legal right to temporary accommodation, and the Scottish Government works closely with local government to assist them in fulfilling their statutory obligation to assist homeless people into accommodation, through funding and other supporting measures."