The clock starts ticking whenever the maximum age is set in the Group Policy Object. However, if individual accounts are set in an organization -- whether to never expire or some other setting -- the Group Policy Object setting in Active Directory will take precedence for all registered accounts. The only exception is systems older than Windows 2000, which don't support Active Directory password management.
Policies can be set for individual workstations or the entire domain, but should be set at the domain level to keep security consistent throughout the network. Active Directory manages password age through the Group Policy Objects editor of its administrative GUI. It can also manage password history, length and complexity.
Password policies, including password age, should be based on a thorough risk assessment of corporate systems and applications. Those with high-risk data, such as customer data or proprietary company information, should have tighter access control policies, meaning more frequent password expirations.
Make sure to have a consistent policy across the organization for password age and expiration, and put it in writing in accordance with corporate IT security standards.
This was first published in August 2008