I don't know, that would be the equivalent of me marrying a twenty-six year old.
Not really. Ten years at sixteen is a lot longer than ten years at nineteen, which is a lot longer than ten years at twenty-five, which is a lot longer than ten years at forty. Human maturation is sort of logarithmic. Also, remember that you're a product of modern Western civilization, and Donna's an Amazon - the culture she's from existed before anyone invented teenagers. Sixteen was adult, plenty old enough to take a throne and go around conquering the known world. Donna getting married at nineteen is actually behind the curve.