Equivalent of Catalan 'Gent jove pa tou' or 'Young people soft bread' referring to being too young for certain tasks

You can say that someone is still wet behind the ears, implying that they haven't finished drying out after birth. This is used to say that they do not understand yet, or are not ready for a task.

According to Wiktionary (but I have not verified the sources), its etymology is from

c. 1850, Pennsylvania, calque from German feucht hinter den Ohren.
From the drying of amniotic fluid on a baby after birth, specifically a new-born farm animal, which last dries behind the ears (partly because licked dry by mother everywhere else). German variants (still wet behind the ears, not yet dry behind the ears, green behind the ears) also borrowed.

According to this clipping quoted in "Green behind the ears": the untold story, by Ben Zimmer at the Language Log, the expression has been an Americanism since at latest the 1870s:

The Newcomerstown "Eye," a new paper, has already got into a squabble with an editor named Persinger, at Bloomington, Illinois. The Bloomington man should wait until the Eye gets dry behind the Ears. — (New Philadelphia) Ohio Democrat, May 2, 1878, p. 4, col. 3

You have to love the tongue-in-cheekness: "The Eye gets dry behind the ears".

The expression “be raw” can suggest the idea:

used to refer to a person who is not trained or is without experience:

• I would prefer not to leave this job to John while he's still a raw recruit/beginner.

(Cambridge Dictionary)

You could say he's as green as grass or green behind the ears (alternative of wet behind the ears).

Green as grass: Completely inexperienced or naive.

Example: The job we had to do was wholly new, and we were all as green as grass.
[Collins Dictionary]

As the name suggest, it comes from green wood that is freshly cut and not yet dried or aged, and is thus not suitable for working yet because it's still wet (inexperienced), so I think it fits the bill.

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Another idiom would be babe/child in the woods.

Babe in the woods: A person who is gullible, naïve, or lacks experience in a specific situation.

Example: Although Jane had always excelled in school, she felt like a babe in the woods when she began attending college.
[The Free Dictionary]

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Never send a boy to do a man's job could also be used.

Never send a boy to do a man's job: Avoid assigning challenging tasks to those that are inexperienced or otherwise incapable of completing them.

Example: I should have known better than to have an intern proofread that letter—never send a boy to do a man's job, right?
[The Free Dictionary]

The old dog for the hard road.

This expression means that experience is invaluable when one is faced with a difficult task. "The case calls for an experienced lawyer, an old dog for a hard road."

It's used to describe a person who has survived many difficulties and has learned much from the experiences.

There are certain tasks that require seasoned judgement and resilience. It would be a mistake, to entrust such an onerous challenge to a novice.

Refs e.g.

https://www.phrases.org.uk/bulletin_board/24/messages/136.html

https://www.learn-english-today.com/idioms/idiom-alphalists/alpha-list_D/id_D6-dog-doghouse.html