How do I create a filter that will return all persons who died before 8 years old?
The search argument is going to be Death => Age => Less than => 8.
The search argument itself will not be a filter, per se. But for example, you can make a group based on that search argument and then you can filter the Index list in the left side panel by the group or you can filter the People List view by the group. Or you can just use the search argument with Search => Person Search (Advanced) => Find to make a list of people matching the search. That list is sort of like a filter.
You can also add the search argument Death => Age => Less than => 8 to other search arguments, for example filtering on the death age and the surname Smith or lots of other things.