In fact, you are correct. In fact, transparent amber does NOT "become opaque" due to aging.

It may become DARKER in color, and this darkening may be compared to or mistake for opacifying..., but fundamentally, opaqueness in amber is due to its cullular structure and the incorporation of gasses that formed microscopic bubbles (when the amber was still fresh resin). Another cause of opacity is inclusions—things with dirt and chaf from the forest (floor) where the trees grew.

Certain cloudy (NOT "transparent") ambers darken with age to the degree that they become much more opaque-looking. The crust they develop is or tends to be thick and rusty-looking. However, diaphenous amber doesn't generally age this way.

You can make opaque amber clear (with heat treatments); you don't get opaque amber from transparent or translucent amber.

Jamey