Reading through Geologic time scale, it defines an age as equivalent to a chronostratigraphic stage, which it says are normally millions of years. But you're right, interestingly the current Meghalayan age only started 4,200 years ago.

It seems all the recent ages are only a few thousand years each (until 2018 the last 10,000 or so were one age, but this was split in three in 2018).

After all that reading I still didn't really understand how they decided that this was a new age.

But anyway, I agree there isn't going to be any difference between 2,000 and 4,000 years so we might as well consider Pompeii fossilised even if not strictly true under the definition. I'm just surprised we consider anything within human history to be a previous geological age, but it seems we do.