It's not that they don't understand it, it's more that it was cool at one point in their lives and became very uncool that now that it's cool again there's still a cringe reaction.

My older siblings will never accept bucket hats or perms for men being cool. Especially not mullets and mustaches.

Canada is kinda different because hockey guys always have mullets because it's the only thing that isn't a buzz cut that looks the same after taking off a helmet, but the mullets were of a different form than current trendy mullets