Perhaps this previous thread may be of use to you.
From what I know, numbers 1-10 are usually written in numeral form, while double-digit numbers are more likely to be written out in letter form. I think it's most important to be consistent. That is, don't mix forms in the same sentence.
My daughter is 10 while my son is nine. (wrong)
I think it's also important to take into account the audience in your writing. Often times when reporting on statistics, it is more common to use the number form. Meanwhile, in more formal writings, it is more common to see the number written out in letter form.
I hope other forum members can contribute their knowledge on this topic.