And--in defence of the commonality of youth and adult taste, you gotta remember. It isn't so much that "Kokomo" would be absolutely invalid to younger demos; more that it wouldn't necessarily be at the expense of older Beach Boys fare. And a lot of that is the consequence of the fact that...even in the 80s, it was pretty hard to escape "I Get Around" or "Good Vibrations" or whatever else, the death grip of boomer nostalgia and the gravity of mass awareness of rock history being what it was. And even if "Kokomo" was a current hit, it wasn't much of a stretch to reach back to, or be exposed to earlier phases of the Beach Boys. And all the more so now that those youthful pop listeners are 20 odd years older and (presumably) wiser. And as for those who'd bluntly reject such earlier BB fare on "dated" or "before their time" or "parent's music" grounds...in practice, they'd be the sort to reject "Kokomo" as well.
So, for better or worse, when it comes to oldies, you won't find the strict "age stratification" of taste there once was. Or if there is, it works against commercial radio in general.