OK I've just had my hair cut and in the trendy city centre salon staffed entirely by people under 30, 3 had grey hair. One was long and voluminous and teamed with doll-like blusher which gave her an 18th century aristocrat in a wig effect, the others were short.
They all looked great.
Obv some young people do have naturally grey hair, but this can't have been coincidence.
I don't know Zippi, I can see it looking better than blonde on older people who currently have unconvincing blonde hair, ie people who are old enough to be grey anyway but won't accept it and don't have a skin tone that blonde suits.
But definitely it works well when teamed with very trendy clothes and I doubt it would work with frumpy ones.