Hi John,
I don't know of anything under the ACL which would prevent this but it may be contrary to state or federal anti-discrimination legislation. Age discrimination is against the law, and occurs when there is a policy that disadvantages people in one age group rather than another – unless they can show that the policy is ‘reasonable in all the circumstances’.
I would approach the golf course and bring it to their attention that they may be in breach of anti-discrimination legislation, and see what they say. If you can't resolve the matter, you can then lodge a compliant with the NSW anti-discrimination board.