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I have a sentence but I'm not sure what to do with it:

"Between the ages of six and ten, they know the difference between reality and fantasy."

My copy-editing question is: do you write...

"Between the ages of six and ten..." or

"Between the ages of six and 10" or

"Between the ages of 6 and 10"

I've been taught that numbers under 10 are spelled out and anything above 10 is written numerically. Do you write them differently if both six and 10 are beside each other in the same sentence?
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The worst thing to do is to mix the word of one number with the numeral of another number.
In my opinion, you should write it as between the ages of six and ten.

I hear what you say about the convention of writing numbers as words or numerals in essays. All I can say is when writing a book, you should not write a number as a numeral unless it's over one-hundred. Unless, of course, it contains a decimal within it. By way of example, you should write the numeral 17 as seventeen. But you would write the numeral 17.4 as 17.4.

It's almost 17 years later, but I'll give an answer anyways.

I was taught that numbers should be spelled out if they are under 12 (e.g. "four," "eleven"). If they are 12 or above, they should be written in their number form (e.g. "12," "26," etc.).

Another person answered with saying that numbers being written as words and in their number forms shouldn't go together; however, that seems perfectly normal, and I, myself, have seen it.

Strictly speaking for your situation, which may be well resolved by now, you would write it out as "six and ten" because the numbers are less than 12. Hope you'll read this lol.