A simple average ??

This is an example of my data and current output:

BoNummer is a unique key.

My current measure is this:

Time from creation to death in Years = 
    CALCULATE (
            VAR TestamentCreationDate = MAX( FactArvesager[Testament creation date] )
            VAR DeathDate = MAX( FactArvesager[Death of testament creator] )

            RETURN

            DATEDIFF( TestamentCreationDate , DeathDate , YEAR ),
            USERELATIONSHIP(DimDato[DatoKey], FactArvesager[Date of start] )
        )

As can be seen it works fine on row level, but I need to find the Average on the 'Date of start' = 53

I think I have complicated things in my head :.(

Plz help