I'm a bit confused by this, you say you're surprised by the younger approval, which seems like an odd choice of words? Does that mean you were still expecting support to be low, but it hasn't been as low as you were expecting? Less dissaproving maybe, if that's what you meant, but I'm not sure they are though.
They you say compared against the older generation which polled agaisnt the monarchy, but the older generations support the monarchy, a lot more than younger folk.
I might have misunderstood what you read, but the support for the monarchy in general decreases as each new generation is introduced, there's been lots of polls on this.
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To be fair to charles he does seem to be a climate advocate and very keen on being green etc (which the young folk will like), but he's not a politician so will have zero control over policy, and his personal thoughts might not outweigh the part of being actually in the royal family to most young folk?