querying the age of a row

Greetings,
I've got a PostgreSQL-8.1.x database on a Linux box. I have a need to
determine which rows in a specific table are less than 24 hours old.
I've tried (and failed) to do this with the age() function. From what
I can tell, age() only has granularity down to days, and seems to
assume that anything matching today's date is less than 24 hours old,
even if there are rows from yesterday's date that existed less than 24
hours ago.

I've googled on this off and on for a few days, and have come up dry.
Someone on a different list suggested that I add a column that get
now() each time a new row is inserted, but that unfortunately won't
help me for all the pre-existing rows in this database.

At any rate, is there a reliable way of querying a table for rows
which have existed for a specific period of time?

--
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L. Friedman netllama(at)gmail(dot)com
LlamaLand http://netllama.linux-sxs.org