When do You Spell Out Numbers?

Any numbers smaller than ten, should be spelled out. Any sentence that starts out with a number should be spelled out ie; four little birds, (not) 4 little birds. Centuries and decades should be spelled out, ie; ‘the sixties’. Percentages should be written out instead of using the % sign. Anything over a million should be written as a word ie; 100 million, (not) 100,000,000.