Of 55,000 chaotic drug users in Scotland, 15,000 or more than 25%, are believed to be 35 or over but they have said services often ignore their needs.
SDF chief executive David Liddell said they have been taking drugs for so long, they have simply given up.
The older addicts, who are known as the Trainspotting generation from the film starring Ewan McGregor, began drug taking in the late 1980s or 1990s.
They account for almost half of all of last year's 574 drugs-related deaths.
Of those deaths, 174, (30%) were among 35 to 44-year-olds and 97 (17%) involved people over 45.
The findings - taken from surveys of more than 70 drug users and professionals involved in their care - show that the more life-experienced group of problem drug users can frequently be overwhelmed by a combination of past experiences and future barriers which impede their ability to overcome their drug dependency.
More than half the older users interviewed by SDF admitted they had had suicidal thoughts.
According to SDF director David Liddell this raises new questions about the true nature of drug-related deaths statistics in Scotland which in 2008 reached two record high levels - a total of 574 for all drug-related deaths, including the new high of 370 drug-related deaths among people known to have had a drug problem.
He said: "We estimate there are about 15,000 older drug users in Scotland, making up about 27% of the 55,000 people with a drug problem in Scotland.
"Yet older drug users aged 35 and over made up 44% (or 162) of the 370 people whose deaths in 2008 were drugs-related and who were known to have a drug problem.
"We also know that long term trends are showing that older drugs users are becoming bigger in percentage terms among the numbers of problem drug users. The percentage of older drug users dying a drug-related death is also increasing steadily.
"What is becoming clear is that older drug users can be badly affected by a range of issues affecting their outlook on life and thus their will to live."