On start up, Uru loads all SDL files and Python files.
It loads the .age files for a hardcoded list of global Ages (things like GUI, GlobalAvatars, GlobalClothing, CustomAvatars).
It reads those .age files, and loads the .prp files listed there by name.
When you link to an Age, it loads the .age file, and loads the PRPs listed there by name.
This is why it is not safe to reference a texture in another Age, because Uru might not have loaded any information about that Age!
The notable exception here is imager static. That is used by the GUI files for the KI, and is always guaranteed to be loaded. So if you're going to use imager static in your Age, think about referencing that global texture and saving some memory/filesize by not distributing a duplicate file