Among 18 to 24-year-olds, only 30% say the monarchy is "good for Britain", compared with 77% among the over-65s.
The survey of more than 2,000 adults in Britain comes as the first anniversary approaches of the death of Queen Elizabeth II.
The pollsters found that overall, 62% want to keep the monarchy.
But they report a "remarkable difference between generations", with younger people much less supportive on remaining a monarchy and more sceptical about the Royal Family representing good value for money.
For King Charles, as he approaches his first year on the throne, 59% of people thought he was "personally doing a good job".
The pollsters say there has been a broadly consistent picture of "overall positivity towards the monarchy", but there is also a sizeable and rising minority who are opposed.
I'm not a fan of monarchy but I see one value in britain's monarchy: they are the keeper of the constitution and protect the people against their leaders abuse in a sort of benevolent dictatorship. They have no interest in weighing on one side or another because that's the only power they have, they cannot gain more. They are, literally, bred and raised for this role.
That's my (positive side) view of it but I'm sure it's totally imperfect and I don't mean that it outweighs the cost and problems it creates