It’s one thing to be intelligent. It’s a whole different can of worms to possess wisdom. And all those times when someone was telling you “you’ll understand when you’re older”—those were moments when they were exercising the latter.
And once folks grew up and understood that all the grown-ups were right—they did understand once they got older—they took to Reddit to share their wisdom and experience in a dedicated thread about the single most “you’ll understand it when you’re older” thing.