Retirement age is 65 for all pilots. There is only a rule about two over-65s not being rostered together.
If you have resigned and want to come back you are welcome to do so provided you left on good terms. If you’ve been away for more than a year you need to do the entire online application and interview again. If it’s less than a year no interview required. QR doesn’t hold a grudge like some airlines who don’t take pilots back. In fact I personally know a pilot who joined and resigned QR three times. Upgrade times currently running at three years as stated.