When displaying timeline information for an Individual, Family
Historian will display facts (events and attributes) from a number of
different records, potentially. Exactly which facts count as
timeline facts is something that can be configured in Preferences
(General tab). This function will return a suitable textual
representation of a given Individual's age at the date of a given
timeline fact. It will return an empty string if called for a
fact that is not a timeline fact for the given Individual (given the
current settings in Preferences).