It's not that there aren't any people whose unsupported assertion is more trustworthy than an explicit, persuasive-sounding argument for the opposite side, though certainly individuals of such discernment and integrity are rare. The issue is that any person so reliable would also necessarily have enough underlying intelligence to be able to, in any situation not involving implausibly extreme levels of time pressure, construct a better (or at least more contextually specific) argument than "you'll understand when you're older." The only plausible explanation for being so vague is if they not only don't want to tell you, but are further trying not to provide enough keywords for you to look up the real reason yourself.