We have always expressed the clinical notion ‘age’ as DV_DURATION. It was one of the first things I remember Sam telling me… ie the duration of time since birth.
BUT there is a clinical situation where you can have a negative ‘adjusted age’ where there is a premature infant, now chronologically 2 months old but born 3 months premature - they are said to have an adjusted, or corrected, age of -1 month. Clinically this is important when related to expected development/behaviour/functional capacity. If they were 3 months premature then you would only worry about their development related to their adjusted age, not their chronological age. By the age of 2 or 3, the difference has usually disappeared and adjusted age becomes less relevant.
So a negative (type of) age, which is clearly not permitted with DV_DURATION, is clinically necessary. And it should be able to be expressed in a combo of weeks, months +/- years.
It makes sense for both the chronological age and adjusted age to use the same data type and I suspect that loosening the DV_DURATION constraints is not feasible.