If you're anything like me, you get confused about which numbers to write out. Is it over 10 or under 10? And when writing seventeen year old, I can't remember if the hyphens belong or not, but I know the hyphens do belong in a query letter. (Seventeen-year-old Rae ...)
Here's the breakdown:
Write out numbers less than 10: nine, eight, . . . two, one.
Don't start a sentence with a numeral: 12 hairless moles should be twelve hairless moles
Spell out (and lowercase) centuries and decades: In the eighties I watched more Dirty Dancing than the average single-digit kid.
Write out ordinal numbers: first, not 1st
If two numbers are next to each other it can be weird. 12 10-year-old girls, for instance, should be written twelve 10-year-
On hyphenation of ages I went for the big guys:
"Hyphenate ages when they are adjective phrases involving a unit of measurement: “Her ten-year-old car is beginning to give her trouble.” A girl can be a “ten-year-old” (“child” is implied). But there are no hyphens in such an adjectival phrase as “Her car is ten years old.”" Chicago Manual of Style
"When the age is an adjective that comes before the noun and modifies the noun, or when the age is a noun, hyphenate." Grammar Girl
If I got anything wrong, please correct me in the comments. Please let me know if you're doing A to Z and I'll come and say hello.