How can i, a teenage boy, masterbate analy occasionally without harming myself and causing future damage such as prolapse?

Masturbation: As far as I know, masturbation is not harmful and I don't believe is causes prolapse but I'm not sure what kind of prolapse you are referring to? Masturbation is not considered to be unhealthy.

By doing it less: By making sure you use fitting glove and enough lubricant to avoid irritation and transfer of germs, you can reduce chances of complications that are mentioned.