There is a couple of different theories posted in forums about alert aging and notification subscriptions. I will try to explain how alert aging works.
During configuration of a new subcriptions you can configure alert aging notification option. You will then get a notification if an alert that meets all other subscription criteria remains in any alert resolution state checked on the alert criteria page for longer than X minutes. For example, if you select NEW under alert criteria and input 10 minutes on the alert aging page you will get a notification if an alert remains in criteria NEW for 10 minutes.
Please note that there will be ONE notification sent to you, not two or three or once every X minute, only one notification, after X minutes.
For example if you have a team of operators watching the console and you want to make sure them don´t miss an alert, so you configure a subscription for second line engineers with alert aging. If your operators dont do anything (for example change resolution state) on the alert for X minutes, you want to send it direct to second line. These second line engineers will get only one notification, it is there for very important that you plan your alert flow. For example don´t send notification e-mails to a personal mailbox, send it to a function or role, which is always supervised.
If you want to escalate your alerts to multiple levels you will have to create multiple notification subscriptions. For example first one after 10 minutes and then a second subscription that will send notification to 3rd line if no one does anything with an alert within 30 minutes.
Pete and I updated our guide on configuration of notification in Ops Mgr 2007. The updated version includes steps for configuration of self-service notification as review of enhancements in the notification engine. You can download the new version here