CPL/ATPL valid or invalid after age of 65 to fly within the country
 

While you have retired pilots flying for Air Transport Service (as per ICAO); What happens to their CPL/ATPL? Are they valid and are they allowed to fly commercially within their country (Specify names of country)

Australia, New Zealand, Papua New Guinea have no upper age limits for pilots holding ATPL and conducting domestic operations. There are quite a few pilots in their seventies flying in all three countries.
However, the frequency and severity of medical checks increases with age, and all three countries impose mandatory flight or simulator checks at six monthly intervals. NZ issues a first class medical for 12 months, but it's only valid for single pilot operations for the first six months. The other two countries only issue a six monthly medical. None of these limitations are a problem for most commercial pilots who are medically fit. The flight check requirement is little or no impost on operators as their pilots are usually on a cyclic or six monthly check regime anyway.
Age discrimination laws apply to prevent a mandatory retirement age; however most operators do have unofficial and carefully concealed upper age hiring policies. To the extent that anyone not already established in a flying job won't get an interview or a start once above a 'certain age'. The occasional exception is PNG if the applicant has extensive prior experience in country, though the national carrier has an upper age limit on hiring expatriates.

Australian class 1 medical is issued for 12 months irrespective of age.

On eyre, you are partly right.

My Class 1 medical certificate has under restrictions "11 Special Condition:" to paraphrase, ATPL not valid six months after exam date unless a further medical carried out IAW CASR Part 67.

This means my CPL etc is still valid, just not my ATPL.