Division of intervals.

Attempting rather unsuccessfully to convert something of the form

45 years 9 mons 5 days 20:28:18.886891

to a total number of months, I attempted

SELECT age(now(), timestamp '1957-06-13') / interval '1 month';

and got the error

ERROR: Unable to identify an operator '/' for types 'interval' and 'interval'
You will have to retype this query using an explicit cast

Does anyone know what interval format intervals are stored as? Is there a way
I can cast this expression to be able to find the number of months within
this interval?

Or is there a function I don't know about capable of doing this? (extracting
the total number of a specific interval from another interval)... e.g. total
number of months, total number of days...

Josh.