A: P is true because of X, Y, and Z.
B: Pff. When you’re older, you’ll realize that P is false.
Being young, I get this one a lot. At some level, it’s an argument to authority: I have X and you don’t, therefore I am correct and you are not.
However, there’s a deeper issue here. One who uses this fallacy is implictly suggesting that the goal of the discussion is to get the parties to agree about what is true, not to actually find what is true. This is a dangerous attitude; would the person assume their beliefs were true if there was no one around to challenge them?
A good way to improve such an argument would be to use it as a form of evidence, not a dismissal of the other party. For example, instead of saying “You’d agree with me if you’d worked in the coal mines for years.” try saying “Well, I’ve worked in the coal mines for years which has let me see firsthand why P is necessary.”
Would you dismiss arguments from a woman, because she "does not know what it is like to be a man"? A woman cannot easily become a man, nor can a young person easily become old. Dismissing their suggestions because of who they are is the height of rudeness.