At first glance I would consider this a worthwhile path to explore; and it meshes well with the comment I made above (before reading this) about the functions are either ignoring or mapping the 29th to the 28th. Subtracting 1 day to leap-year dates effective does both. I'm sure there are other interactions involved here but if for someone willing to do the work this approach seems to have some merit.
And given that simple of a definition writing your own age function that checks for the 29th and makes the adjustment would be trivial...so if that's the extent I'm not sure PostgreSQL would want to change the definition of the included age function and risk backward compatibility issues.
