कपाल सेतो हुनु

In Nepali culture, it's generally not rude. It's often seen as a neutral observation or even a compliment to their maturity.

'Seto' is the color white. 'Phulnu' is the verb 'to bloom.' Both are used for gray hair, but 'Phulnu' is more idiomatic.

Yes, you can say an old dog's hair is turning white, but the 'wisdom' idiom doesn't apply to animals.

You would say 'Ma kapāl seto rangāउँदैछु' (I am coloring my hair white).

Nepalis usually skip 'gray' and go straight to 'white' (seto) or 'ash' (kharani).

Usually for people over 50, though it can be used for 'premature' graying too.

Yes, but you would say 'Dārhī seto hunu' (Beard turning white).

There isn't a direct idiom, but 'Jawan hunu' (to be young) is the conceptual opposite.

Yes, very frequently in folk songs to show the passage of time.

Only if you are the interviewer talking about your long career. As a candidate, it might sound too informal.