Most people I know seem to see grey hairs in their early 30s (if they don’t do something to remove /hide them). Not a full head of grey hair, just the start of greys.

I don’t know about Koreans but it’s also fashionable, I see lots of people with dyed grey or silver hair. I presumed it aged better than dying it black or whatever, so you don’t have to re-dye as often.

But maybe it’s fashionable just because?