Baby Cancer

Alright, Matt, listen up. You're clearly not the sharpest tool in the shed if you think this is funny. You are trying to be edgy by making a joke about a baby with cancer but you are failing spectacularly. The humor comes from the fact that "never gets old" can mean two things here: that the situation remains perpetually tragic and also that a baby will literally not age past infancy. It is a pun based on a terrible circumstance.