CGT on sale of small business owned for 13 years by person over 55. Is there a 100% exemption in CGT on the sale of a business if sold for under $500,000 in NSW by a 57 year old owner who has owned the business for the past 13 years?

Kirsten,
The ATO website provides the answer to your question in full detail.
15-year exemption (from ATO website)
If your business has continuously owned an active asset for 15 years and you're aged 55 years or over and are retiring or are permanently incapacitated, you won’t have an assessable capital gain when you sell the active asset.
The concessions are available when you dispose of an active asset and any of the following apply:
•you're a small business (that is, have an aggregated annual turnover of less than $2 million)
•your asset was used in a closely connected small business
•you have net assets of no more than $6 million (excluding personal use assets including your home to the extent that it has not been used to produce income).

Your accountant will be able to assist you with this.