Sentence starting with a number

• 71 people were rescued from the sinking ferry by the Latvian helicopter crew. (This is not wrong, but it is untidy.)
• The Latvian helicopter crew rescued 71 people from the sinking ferry. (This re-worded version is tidier.)
• Seventy-one people were rescued from the sinking ferry by the Latvian helicopter crew. (In this version, the number has been written in full to avoid starting the sentence with 71.)

Above are the examples from grammar-monster.

Are the guidelines sound, and backed by a recognised authority?

Is there any limit to the number, I mean, does the same rule apply to the following sentence:

7100 people were rescued from the sinking ferry by the Latvian helicopter crew.

Solution 1:

I learned that for numbers, if it is less than 100, you should type it out in words. If it is 100 or more, put it in numeric form. I also learned you should never start a sentence with numbers, so re-wording to get the numbers later is preferable. (It also means you can start a sentence with the worded forms of numbers less than one hundred.)