Of all the prejudiced, dismissive remarks and thought-terminating cliches I've ever been exposed to, none has stoked the fires of my righteous outrage more effectively than "you'll understand when you're older".
Even such gems as "because I said so" can't manage to match the level of dismissive arrogance as "you'll understand when you're older".
Implicit in the statement is the absolute certainty that whoever you're talking to is inherently inferior to yourself. That the gap between you is so great that there is no possible way you can explain your side of the argument to the other person, because they would be completely incapable of following your inherent brilliance.
Make no mistake. This is not about experience. That's what "when you have kids, you'll understand" is about. "You'll understand when you're older" is a statement that denies any experience could possibly make the slightest bit of difference. That there is nothing you can do about this inherent inferiority you have to the other party that could possibly bring you up to their level.
And of course, it's a go-to excuse for stupid people who don't have an actual argument to pretend that the reason they aren't presenting one is that they're actually far too superior to the other party, not the truth, that they have no argument that could stand up to the slightest bit of scrutiny.
There's an old saying among scientists. "A theory that explains everything, explains nothing." In other words, if you can use the exact same argument for any position at all, then it isn't an argument that's worth anything, because it's just as effective at arguing for falsehoods as it as at arguing for truths. That's the reason lists of logical fallacies get linked to so often on debate boards.
Still, never let it be said that I refuse to test the assertions of others against the objective standard of reality, no matter how much I disagree with them. I was told quite often when I was very young that all the age-based discrimination I noticed and complained about was something I would "understand when I was older". I'm older now. I have a better vocabulary for expressing my views, and I've got a broader education in the social justice struggles of yesteryear. But all those things I said were wrong back then, are still things I say are wrong today.
The truth, I think, lies with an absolutely wonderful quote from Albert Einstein. "If you cannot explain something to a six year old child, you don't understand it yourself."