I just went through and fixed this in my latest manuscript when I submitted it to a publisher that had guidelines that specified this. First time I ever thought that hard about it. The guidelines said to write out numbers, thirty instead of 30, except for really large numbers, I think. Like years - 1976 is okay. I misplaced the guidelines, the publisher rejected my manuscript anyway, and I can't remember precisely off the top of my head. I do recall something from my newspaper days that said sort of the same thing, that you write out any numbers below 100.