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.
It was often said people get more reactionary as they grow older because they start gaining assets that a naughty Labour government could steal (I'd you read the Tory press) but: a) I didn't observe that in my Dad and his friends and b) those younger generations are looking at never getting those assets. The latter might just give up on thinking they can change things but they're largely going to be disaffected by The System for most of their lives.