To determine character age, use CharacterAge in export_descr_character_traits.txt but not as a trait...

;------------------------------------------
Trigger general_ages1
WhenToTest CharacterTurnStart

Condition AgentType = family
and CharacterAge > 15
and CharacterAge < 40
and Trait AgeingGeneral < 1

Affects AgeingGeneral 1 Chance 100

I've got this working in my bug-fixer 3.36/GAPH Modded RTW :)