I am trying to understand the age function in Postgres and this is the code that I encountered – https://github.com/postgres/postgres/blob/REL_15_6/src/backend/utils/adt/xid.c#L98

I tried to understand this and these are my understandings –
The age of an a given xid [input param(0)] of the function is

pg_current_xact_id() - xid

However can this not be negative?

x_min for a transaction could be 1000. Post that a transaction wrap around occurred and thus the current transaction id pg_current_xact_id() is now 100.

So if I want to calculate the age of the transaction whose x_min is 100, I would get age to be negative.

Initially in postgres, during a freeze, x_min used to be set to a default FrozenTransactionId which was 2. However that is no more the case in newer versions of postgres.

Is negative age allowed?

I am not able to test this locally and bring a transaction wrap around