Like many things in writing there is a disagreement on to when to spell out your numbers.
There is the opinion that everything under 100 should be spelled out, while everything over should be numerical.
The other opinion is that all numbers that are one word should be spelled out, while everything else should be numerical.
With either of these 'rules' or 'guidelines', you need to spell out all the numbers from one to twenty.
So she wasn't a 5'6", blonde, 16 year old. And he wasn't a 6'4", brunet, 18 year old with 8 inch dick that was 2 inches wide. (I know it isn't the greatest example, but almost every story I've run into on here has something like this in the first few paragraphs or before they have sex.)
She was a five-foot-six, sixteen-year-old, blonde and he was a six-foot-four, brunet. At eighteen-years-old he had an eight-inch dick that was two-inches wide.
I prefer the under 100 'rule' and that is the one that I follow. But it would make a lot of writing a lot better to not see random digits in it throughout.
At 25 she ran a 24 hour restaurant and never got enough sleep.
At twenty-five years old, she ran a restaurant that stayed open all night and even holidays. Never getting enough sleep, she had no time to even think about sex. (When you take out the numbers and try to say the same thing in a different way, you often find something better.)
At 12 that night, he met her in the parking lot.
At midnight, they met in the parking lot.
She called his cell 3 times before his lunch break the next day, and when he got home he had 7 messages from her on his answering machine.
She called his cell three times before his lunch break the next day, and when he got home he had seven messages from her on his answering machine.
How do you feel about spelling out fractions, decimals, equations, street addresses, numerical highway designations, flight or train numbers, populations counts, sporting festivities and my favorite - Movie Titles.
Would it be: Rocky Fifteen, for instance?
The same GQP demanding we move on from January 6th, 2021 is still doing audits of the November 3rd, 2020 election.
I mostly write out my numbers because it feels more proper, but there are a few instances of numerals in my writing. An example I looked out: "a 24-pack of [beer]" seemed more natural to me than writing "twenty-four-pack".
I would never stick with any hard and fast rule like 'everything under 100 in words'; I just go with my gut. It could be viewed as a literary technique and depend on how you want it to be read, e.g. I think there's a difference between "she was a hundred and two years old" and "she was 102 years old", if only in the reader's mind.
There are lots of issues to explore here (most of which Jeff pointed out) and, in most cases, there's no right or wrong answer. Sometimes Roman numerals might be appropriate.
A comment on height: 6'2" would be read (by most people) "six foot two". Technically, in written English, one should include the word "inches"; otherwise it doesn't make mathematical sense. I'm wary of this, although it's a minor point.
Direct speech is an area I do have a rule for: always write it out! A person doesn't say, "Let's meet at 2:15." They say, "Let's meet at two fifteen (or quarter past two)." Similarly, they don't say, "There are 2000000 people in this city." (They say "two million".) This is because they are actually saying the words, not the numbers. People might say numbers differently but if there are quotation marks then it's exactly what they said. I could probably even find a few exceptions to this as well, though.
for addresses, phone numbers and such, i use numbers. if i'm describing a scene and the number is small, i spell it out, and if there's a lot of numbers involved, i try not to be specific so i can spell that out too (There were four guys looking for the house at 4884 Elvis Boulevard. Four among thousands hoping to get tickets for the big event.)
You can’t truly call yourself peaceful unless you are capable of violence. If you’re not capable of violence, you’re not peaceful. You’re harmless.