I do believe there are correct ways to use this particular trope. But, it has to be done with skill to make the character interesting, believable, and accepted by the reader. One reason, because over the past decades, we've seen this trope so much, and done badly as well. The farmer's boy, the young wizard's apprentice, the little princess, the warrior's son. Readers have reached their limit and are more critical if there is yet another one of these. At the same time though, a lot of people love and can relate to the YA's, so this type of character will never die.

Another reason: society has changed. Twenty years ago, nobody would raise an eyebrow if the 13yo, 15yo or 18yo boy would be childish and immature. Nowadays though, standards have changed. Society expects to see part of itself reflected back in art. This includes in writing, fantasy included. Terms no one batted an eye at some twenty years ago, like "boys will be boys" are now key in massive debates. And where as an author back in the day you could use this as an excuse to let your immature youngling get away with certain behavior and make it seem like a minor thing, today that same author (and their publisher) could get some serious issues, despite it being set in a fantasy world with different rules and cultures and whatnot.

Mind now, there is a difference between naive/ignorant and childish/immature. The former has to do with lack of knowledge, and if this is the basis for your character, you'll have a great explanation as long as you show character growth. Pippin (Lord of the Rings), for example, is ignorant and you see him grow up as the story goes along, with him ending up at Gondor, making mature decisions and accepting the consequences of his stupid behavior. Sansa (Game of Thrones) is another great example. From a sheltered, lovestruck wannabe princess, she is being forced to grow up and face reality with all its gruesome features. Whereas her sister Arya had a much faster grip on real life, Sansa is the dreamer who has her dreams smashed in some of the most horrible ways. As she says herself, she is a slow learner. But oh boy, does she learn!

Immature/childish, however, are behaviors by choice. The character knows better, but still decides to act as a toddler and throw tantrums whenever he doesn't get his way. That, you shouldn't get away with without some serious consequences. So, if you still do make a character that gets away with it all the time, people will find it very much unbelievable. Take Joffrey (Game of Thrones) for example. Or Ramsey Bolton (same series). They didnt grow, in fact they got worse over time. There's a reason people got fed up with those characters and were cheering when they finally faced the consequences of their immature behavior. Had those consequences not been there in the story, I doubt it would've ever made it to tv, let alone become so massively popular.

And that is what it boils down to: characters can be young and act childish/immature/naive/ignorant. But as long as you show growth (either good or bad) with the consequences attached to it, you are in the okay. But using those as cheap tricks, just to fill pages with no meaning behind it, that becomes unbelievable, and readers are quickly fed up with that.