Throughout the series we see that Mom and the Peofessor worked together and had a relationship. This would make Mom either the same age has the Professor, or older (she's looked as ancient in the flashbacks as current time). So why hasn't Mom been summoned to the big retirement home in the sky?
Out of universe: never directly addressed. Most likely, a result of Mom having been introduced very early on, while The Near Death Star was introduced somewhat later.
In universe: never directly addressed. Speculating: Mom is very rich and very powerful. It is difficult to force such people to do anything they don't want to do. Further, as Kevin Milner points out in the comments, the robots who enforce the mandatory retirement age are most likely manufactured by Mom's Friendly Robots. As we see in the episode Mother's Day, this gives Mom considerable influence over them.