Strictly speaking, there is no right or wrong answer to this question -- it's entirely a matter of style. Some style manuals call for using numerals as much as possible, while others use them as little as possible. Check to see if your office has a style guide and if it does, follow that style.
Many style guides call for writing out the numbers zero through nine and using numerals for 10 and any number higher than that up to 999,999. After that, you use a numeral and the wordmillion, billion,etc.: Scientists believe the universe was formed some 15 billion years ago.
An important exception: Never start a sentence with a numeral:Ten times he tried to call her. On the 11th try, she answered.
Also, ratios take numerals:About 1 out of 3 therapists surveyed disagreed. Sports and other scores also take numerals:The Tigers beat the Indians 5-4.There are a few other exceptions.
For currency, most style guides would opt for $20,000, not twenty thousand dollars,but again, check your office's style rules to be sure. Write out a dollar, and for amount less than that, use the word cents following the zero-to-nine rule: four cents, 13 cents.
According to most accepted style rules, you do not use letters such at th, st,ornd after a date. The correct form for a date in the US is February 14, Feb. 14, 2007, or February 14, 2007. Accepted British style and that of the US military is 14 February or 14 February 2007.