I would generally only say "good afternoon" if I answered the phone in the afternoon, or if I was starting a formal speech (e.g. Good afternoon, ladies and gentlemen).

It's not that it's old-fashioned, it's just a bit of a mouthful. I do say "good morning" when I come into the office in the morning.

If I was greeting someone in the afternoon, I would just say "Hi" or "Hello".

This has nothing to do with whether you are a millennial, either. (I am not a millennial; I am in my late 40s.)

I rather like the Australian shortening of afternoon to ARVO, but it would probably get blank stares in other English-speaking countries.

I would generally only say "good afternoon" if I answered the phone in the afternoon, or if I was starting a formal speech, e.g. "Good afternoon, ladies and gentlemen". (I live in England.)

It's not that it's old-fashioned, it's just a bit of a mouthful. I do say "good morning" when I come into the office in the morning.

If I was greeting someone in the afternoon, I would just say "Hi" or "Hello".

This has nothing to do with whether you are a millennial, either. (I am not a millennial; I am in my late 40s.)

I rather like the Australian shortening of afternoon to ARVO, but it would probably get blank stares in other English-speaking countries.