The date of manufacturer is on the left or right tank of the rad. (If you want to know)
It is a good idea to change the rad & expansion tank every 5 years. If the car slightly overheats, it warps the head, and pulls our the cylinder head bolts.
E39 are like Princesses, they demand a lot and don't take well to getting old.
My cooling system is 4 years old, and will be replaced next year...