The number of Baby Boomers - who grew up in the Sixties - getting spaced out on cannabis has almost doubled in the last eight years.
A study of more than 47,000 over 50s in the US found a 71 per cent rise in marijuana use between 2006 and 2013.
Dr Joseph Palamar of New York University said: "We found only five per cent of these older adults felt using marijuana once or twice a week was a great risk to their health. I thought the perception of low risk was fascinating because, typically, we think of older generations as drug adverse, and perceiving most drugs to be risky.
"But apparently very few Baby Boomers consider marijuana use risky. But after all, this was the generation who was there, in the late 1960s, when the counterculture revolution exploded marijuana into mainstream popularity."
The finding - published in Addiction - mirrors previous research in the UK showing recreational drugs are being increasingly taken by older Britons.
Researchers said the Baby Boomer generation reported higher rates of substance use than any preceding generation.
Co-author of the study, Dr Benjamin Han, said: "With the increased availability of legalised marijuana, there is an urgent need to understand the prevalence of its use and also its effects among older generations.”
Over 65s had a significantly lower prevalence of marijuana use compared to those between 50 and 64, but this increased two and a half times over the eight years. Overall, use was higher among men than women. Most marijuana users said they first started before the age of 18. This means that most of the current users either continued use or have begun using again more recently.
But the researchers believe the population may be at a particularly high risk for adverse health outcomes, as multiple substances like marijuana, prescription drugs and even illicit drugs all used in combination may make older adults vulnerable to poor physical and mental health.
Dr Palamar said: "For years we have been worried about the potential effects of marijuana on the developing brains of teens, but now we may need a bit more focus on their grandparents, who are increasingly more likely to be current users."
Added Dr Han: "Older people may use marijuana for a variety of reasons, including medical reasons. However we need to make sure they are not using in a hazardous manner since older adults may be vulnerable to its possible adverse effects.
Earlier this year an analysis of a series of UK national surveys found among those aged 50 to 64, lifetime use of cannabis increased more than tenfold, from one per cent in 1993 to 11.4 per cent in 2007.
Recent use had multiplied by a similar amount, from 0.2 per cent in 1993 to two per cent in 2007.
And there's growing evidence to suggest this extends to prescription drugs as well as illegal narcotics, with escalating misuse of pharmaceutical drugs such as painkillers or sleeping pills.