Working on some resumes, it occurred to me that educational establishments and qualification boards must have award and grade records going back decades; and town archives must have to keep track of everything not just for historical purposes but as useful records.
What I mean in the main question by ‘practical purposes’ is to exclude purely academic historical and archaeological research.
I imagine that the answer to my question will be century-old birth records and old land claims.
In the UK, it's probably the records used by heralds of the College-of-Arms although lawyers who dig through old statutes might use older records.
In the US, Sanborn maps and survey maps from the 19th century are used in environmental litigation to determine when geography was changed.
Note: I don't know that this is oldest, but how can you bubble sort without data in your array? :)