Hi,
Matching a slot (in a deftemplate) within a rule requires you to state the deftemplate and the slot you wish to match against.
You can test the age slot in immunizations with: (immunizations (age ?age&:(< 180 ?age)))
See 5.4.1 Pattern Conditional Element in CLIPS Basic Programming Guide[1] for more information about why the "?age&:(< 180 ?age)" is required...