From
Hart's Rules for Compositors and Readers, Oxford, 1986, the following:
Words or Figures? Figures are to be applied generally to all units of measurement (tons, cwt., feet, seconds, and so on). Separate objects, animals, ships, persons, etc., are not units of measurement unless they are treated statistically.
e.g. "The peasant had only four cows." but "A farm with 40 head of cattle." In the first example each cow is identifiable individually.
In descriptive matter, numbers under 100 to be in words; but print "90 to 100," not "ninety to 100."
