Is there any published text that, in effect, states that level increases, inherit bonuses and age bonuses stack onto the new mental ability scores resulting from PAO?
The bonus gained by polymorph any object is untyped, and you're still under the effects of those other things, and bonuses of different types (and untyped bonuses from different sources) always stack.
And, IIRC, they apply in the most beneficial order.
Or am I wrong?
The change in your ability scores from PAO isn't a
bonus at all. It
replaces your ability score with that of your new form.
It would indeed stack with enhancement (and other) bonuses that you still benefit from, as long as you still receive those bonuses in your new form, though. Now... whether aging modifiers and inherent bonuses would carry over is another thing... since they're arguably enhancing your old body, and not your new one. You get the stats of an
average member of your new form, not those of a
venerable member of that species...