While vowel sounds aren't usually a problem for children learning to speak, consonant sounds continue developing well into the early primary school years.
The latest-developing sounds are usually "v" and "th", and they are mostly acquired by age eight.
However, "z", "r" and "s" can also take a while, and even "l", "sh", "ch", "j" and "zh" as in "treasure" are often still firming themselves up in the first year or so of school.
So we have an obvious problem here, in relation to learning literacy.
How are children meant to learn about sounds and the letters/spellings that represent them, when many of them can't even say all these sounds in their ordinary speech yet?
The human critical language-learning period
Our education system doesn't understand language very well, frankly.
The young human brain is wired with special language-learning equipment, which allows little children to suck language in like vacuum cleaners, and learn it at an incredible rate. For more detail on this, see the Ted Talk "The Linguistic Genius of Babies" by Patricia Kuhl.
This equipment works best up to about age six, by which time most children have learnt the basic grammar and vocabulary of the language(s) they hear around them.
By about eight, they have fine-tuned the sounds and are able to understand and construct complex sentences, express abstract ideas and use language for reasoning.
In evolutionary terms, most of their language-learning equipment is then obsolete. The brain still needs to learn new vocabulary throughout life, and other fine-tuning of the language system goes on till adulthood, but by about eight, most of the language house has been built.
Brain cells are fuel guzzlers, so it's not a good idea from an evolutionary point of view to keep brain equipment running after it's required. So somewhere in the early primary school years, the language-learning apparatus starts to turn itself off. By about puberty, it's pretty much gone.
Wasting children's golden opportunity to learn languages
After age eight, and definitely after age 12, it's much harder for learners just immersed in a new language to pick it up. They usually need to attend classes, study verb structures and otherwise apply conscious effort to learn it well. They learn as second language learners, whereas a child learning a language before age six does it as a first language learner, with the necessary equipment to do it fast and well.
Even spending time communicating with someone who speaks another language imperfectly is very worthwhile before age six, as children don't just parrot the language of the adults around them, they take it apart and reorganise it in their heads according to universal grammar principles.
No parent says, "You runned fast" to a child, but children typically go through a period of saying things like "runned" and "taked" and "goed", because they are learning a past tense rule and over-extending it. This is clever and shows they are not just little parrots. They learn language by applying their language-learning equipment, and can do this even from imperfect models.
Parents who say "my Italian isn't very fluent or correct, and I don't want my child to learn incorrect grammar, so I'll wait for her/him to learn it properly at school from a fluent speaker", fundamentally misunderstand the nature of language and language-learning. By school, it's already too late to really capitalise on a child's most golden opportunity to learn another language.
So what does our education system tend to do? Wait till children's critical language-learning period is drawing to a close, or over, and then introduce foreign languages!
I started learning French in secondary school, at age 12. My entire preschool and primary years, when I was wired to learn languages, were spent in solely English-speaking environments. I hardly had an opportunity to listen to foreign languages on the TV.
As an adult, I can barely speak French at all, despite having studied it for five-and-a-half years. When I wanted to learn Spanish in my 30s, I had to do it the hard way, with lessons and textbooks, and I still need the subtitles during Almodovar movies.
Taking advantage of the critical language learning period
If our educational system understood language better, we would emphasise learning spoken languages from birth to about six, while our brains still have high-powered equipment to do it fast and well.
After that, written language could be introduced, starting off with the sounds/letters that are in everyone's repertoire, and leaving things like "v" and "th" till earlier-developing sounds were covered.
Of course, the process of learning a letter/spelling feeds back into the process of learning and consolidating a sound, so children wouldn't need to wait till later-developing sounds were 100% correct before meeting them in print.
In fact once children have the general idea of words being made of sounds, and letters/spellings being how we draw these sounds, and a decent repertoire of both sounds and letters that they can use for reading and spelling, meeting later-developing sounds in print would probably help these sounds become consolidated in their speech.
However, teaching a five-year-old who can't even say "l", "sh", "ch" and "j" properly yet to start off their reading and spelling career by memorising the words "the" and "of" (with irregularly-spelt "v"), before they can read and write words with early-developing sounds like vowels, p, b, t, d, m and n is, I think, highly problematic.