It isn't based on individual age. I know Korean cousins with a 20 year age difference where the younger one gets the honorific because his lineage is older and he's an earlier generation. They never speak Korean with each other.
It is. you are using a special anecdotal case in a family situation (that same person would use honorifics towards their friends who are much older) to negate the rest of Korean society that uses one's age to decide who is above and below you, and the necessary honorifics. DK effect is quite laughable to see from a non-Korean.
I invite you to address your professor or sunbae or the elderly without honorifics and see their reaction. You wouldn't be able to use your race card very long.
I have seen low-rank white dudes get status upgrades in other Asian countries for a variety of reasons. One architect told me how he was hanging with high status Indonesians, and how he could name-drop NZ politicians (for example, the minister of finance) because our culture means low-rank citizens can personally know people in high-rank positions. Perhaps I can manipulate the Korean status game in my favour because my background is somewhat unmeasurable.
I'm many cases this would apply even if you are Korean but grew up outside the hierarchy (i.e. grew up in the US)
You'd have some status as a us citizen as a white person, but it's complicated and a double edged sword
It might have worked in the 60s or in South East Asia but certainly will not work anymore in East Asia, especially in a hyper connected world. In fact many foreigners who achieved celebrity status by lying were quickly cancelled.
By large, a non-asian person in East Asia is largely limited to an exotic animal at the zoo. Somewhat of a novelty for kids and adults to point and laugh "wow! we have foreigners in our country speaking our language".
You would see the same response towards a monkey suddenly speaking Korean or Japanese.