> “Reasonable” is a lynchpin bearing an awful lot of load here.
No it's not. I could imagine sentences where it would be, but not this sentence. Here, watch me replace the word:
"99% of adults are not allowing their teenager to to experiment with heroin or giving their 12 year old permission to drive their car down the freeway."
Even if most people aren't ""reasonable"", they are whatever adjective that sentence describes.
“Over half of U.S. adults surveyed said that it’s very inappropriate, somewhat appropriate, or were uncertain whether it’s appropriate for their child to set boundaries for their interactions” is a reasonable-sounding statement, too; it’s plausible, applies to children of all ages (below or beyond age 25!), and is demonstrably an aspect of culture represented by media and other ephemera.
The position itself is, of course, completely unreasonable — boundaries are never inappropriate to consider (and to contrast with the parent’s boundaries about immediate versus deferred conversations in unsafe circumstances, the child’s age and cognitive ability to assess risk, and so on), no matter how uncomfortable it is to teach a child about boundaries by honoring one they’ve presented one! — but that intolerance is presented in such a reasonable guise, with a tone of majority support to quash any brief qualms, that it causes many to overlook its true nature.
See also “pleasant”, as in “Pleasantville”.
If you thought the discussion was actually about 12 year olds doing heroin and driving, I think you might have missed something.