Basic Book Design/Writing Out Numbers
The Chicago Manual of Style (8.3) advocates spelling out whole numbers from one through one hundred. However, this makes text less readable (and also longer). Few people see ninety spelled out, but everyone sees 90 often. E.g., advertisements say "90 Days Same As Cash!"
I suggest spelling out whole numbers one through twelve, and multiples of ten, through one hundred. Use numerals for all other numbers.
Where you switch from written numbers to numerals is unimportant. What counts is being consistent. E.g., don't write "thirteen" on one page, and "13" on another.
Exceptions include quotations (use what was written in the original), money, percentages (e.g., 70% is easier to read than seventy percent), and when a sentence has two or more numbers (e.g., "The average woman moves in with a man at age 20 or 21.").