I am making a reincarnation novel with all of the protagonist's memories intact from the previous life. So what is the time when the protagonist can actually communicate through speaking?
There was a story recently about a man who'd had an injury, stroke or similar and needed to learn to walk and talk again, just as his partner had a baby. Hopefully someone can find it as his recovery time is effectively the answer.
Even though he had the knowledge of these activities, he still needed to go through the same motions, lying on his back and watching the limbs as he attempted movement, making strange noises as he relearned how to control his vocal chords. In doing so it's said he reduced his recovery time below that of people who get frustrated by their inability to pronounce fully formed words for a considerable period.
It's not just about learning the words, it's about making the connections between the brain and the peripherals and learning how to control them, and building up the muscle strength and control to the level of finesse required for speech.
If this is his first time reincarnating, it may be no quicker than the average baby. If he is an experienced reincarnater then perhaps he has a set of exercises down pat to accelerate effective use of vocal chords and other muscle groups.
I don't see what is their difference with normal child therefore I think it would all go along the usual language acquisition period. The brain and muscles have to develop enough to be able to articulate the sounds.
there is a "sensitive period" of language acquisition in which human infants have the ability to learn any language. Several findings have observed that from birth until the age of six months, infants can discriminate the phonetic contrasts of all languages. [...]
At a very young age, children can already distinguish between different sounds but cannot produce them yet. However, during infancy, children begin to babble. Deaf babies babble in the same order when hearing sounds as non-deaf babies do, thus showing that babbling is not caused by babies simply imitating certain sounds, but is actually a natural part of the process of language development. However, deaf babies do often babble less than non-deaf babies and they begin to babble later on in infancy (begin babbling at 11 months as compared to 6 months) when compared to non-deaf babies.
I think it would start from about 1 year onward.
It's around three months.
At birth the human larynx is in the normal, animal location, enabling babies to nurse without risk of choking. The larynx typically begins to move lower at about three months of age and reaches its final position by age four. People familiar with children’s speech will notice that the start of the relocation is also when infants start to coo. The end is about the time the children finally become clearly intelligible to well-meaning strangers.
The movement of the larynx is primarily evolved to support our need for speech. Complex speech patterns require the valve to select between breathing air and eating food to occur lower than it does in other animals. The result is that it is much easier for us to choke on food until we get the hang of controlling this valve. Infants have a high larynx so that they can nurse without choking. As they get older, and gain more control, the larynx descends to start providing us access to the full repertoire of the human voice.
An adult's soul trapped in an infant body? In this realm of imaginate, it is possible that the personality of the adult, possessing all knowledge and emotions of its previous life, will push harder the body to respond and develop adequate brain capacities. You will probably have a funny-talking, well-learnt 5 y.o.
The idea of a child being reincarnated with vivid memories of their previous life is not only not that far-fetched, it isn't even fictional. There are lots of real-life stories of exactly this. These children, usually toddlers, have demonstrated intimate first-person knowledge of someone else's life. One child's vivid nightmares actually solved a murder case that had long gone cold.
These all follow a rough pattern: the child starts discussing these memories at around one-and-a-half, the age when a child is first able to put words together into rudimentary ideas, and the memories fade by age four or so as new ones take their place. The reason they don't discuss them as an infant is that simply having the memories is not enough; an infant's brain just doesn't have the ability to move the right muscles in the right way to form words.
(One reason for this is that the neurons in a baby's brain have not yet developed a sufficient myelin sheath, which is a fatty coating that insulates it from the neurons around it. Without that, the electric charge surging through a neuron when it fires also triggers all the ones around it; which is why a baby's movements are so sporadic.)
If you could somehow implant an adult brain into an infant's skull, shrinking it down to fit, theoretically he or she could start talking within a few days (after recovering from birthing shock); all the moving pieces are there, they just need to grow. But barring that, you're basically looking at the same developmental timeline that all children go through, which is that they start forming words at about a year old and basic sentences at about two.
I have seen an instance of a child verbalizing in recognizable words and phrases at 6 to 8 months. However, I have seen several children who can do that in the 12 to 15 month time frame. However, this was not on the order of the fluency of talk show material. It was recognizable thoughts beyond "Ma" or "Dada."
This is just a couple of subjective singular data points.
While I am a scientist, I am not a medical or developmental scientist. You might ask someone mainstream in the field, like a speech pathologist who has an interest in very early speech development.
You're asking about radically different things.
Age to communicate via the vocal cords? Zero.
All (normally developing) neonates can communicate via the vocal cords. Screaming, etc. Initially, there is basically one word: FIX-ME!!
As infants develop, they quickly are able to differentiate their screams/vocalizations. Some do this more than others and of course it's dependent on how well caregivers pick it up. One cry can mean "I'm hungry," another "I'm wet/need to pee," another "I need to be held."
Age to articulate spoken words such that they are understandable by others? 4-6 months (though for most children, it's 11-13 months).
This is fairly rudimentary and it depends both on the word and the listener. Most infants do not have the cognitive skills to really create language here. For them, these articulations are practice sounds. Even deaf children do it to some extent.
While it's not super common, infants can have real spoken words in this period. My own daughter had her first spoken word "all done" (yes, that counts as one word) at 5 months. My husband heard it from the other room and yelled "did she just say 'I'm done'?" Naturally, people didn't believe us. Until they heard her say that and other things themselves, then they all believed it.
Age to communicate using symbolic language? Very young.
What do I mean here? I mean actual words in an actual language. But spoken words need some time due to maturation of the vocal tract. So the words come from sign language. Note: if a caregiver teaches a child a made-up or wrong sign (something no local signer would ever use, or one that is misarticulated), and the child uses that, it still counts as a word, the same way teaching a child a made-up spoken word does; output comes from input).
Since I have a background with sign language (including teaching it to special needs students), I had already planned to teach it to my baby. But it turned out that "baby sign language" was a huge fad when my daughter was born (maybe it still is). I refused to use the made-up words but instead used American Sign Language (ASL) adapted for the developmental level of her finger articulation. I only used single words and no grammar.
And this is where I lament the fact that I didn't own a video camera (smartphones didn't exist yet but I could have bought video and I was stupid not to). Because no one hearing this story believes me (even seeing the still pictures). But dozens of people who saw it in action were instant converts and babysitters could interpret her needs via signs.
My daughter started learning a couple signs at 2 weeks. By 2-3 months, she had a vocabulary of several words, quickly growing to maybe a dozen (I took careful field notes but am doing this from memory). One of her common words was "all done" which she used while peeing/etc either on the toilet (after she could sit up and with a special seat and adult help) or being held over the sink. This is the word that morphed into her first spoken word (she said it while she signed it, something we modeled to her). (Our experiences with this were filmed for a documentary about Elimination Communication but were cut from the final version, though I have a copy somewhere.)
Age to have actual conversations? At least 1, up to 3 or 4.
This varies by child a lot. I know a lot of children who could simply not sustain a back and forth conversation until they were 3 or 4. After that, they were fine. A lot of kids can do it at age 2 and many can do it earlier. For my child, we were having fairly complex conversations at 18 months (I remember in particular because one of our cats died then and she was asking about the afterlife, talking about seeing his ghost, and all sorts of things to freak parents out).
What about memory?
The memory issue is completely separate. While there are stories of people who can remember things from birth or infancy, it's hard to know what is true. Memories fade from young children.
Using my daughter as an example again, she had a terrific memory when she was very young. She could remember things from 18 months when she was 2, for example. But then it all faded. She lost all her sign language very early, by age 2, because we stopped using it with her (no need). Starting about age 6 (or so), she stopped remembering a lot of things she had known, even things she knew continuously. There is a veil that goes over early childhood, perhaps as a consequence of brain development. My daughter is 13 now and remembers kindergarten well, and a trauma that happened at age 4, plus a few things from earlier. But pretty much all her memories from birth through at least age 3 are completely wiped out.
My background: I have a B.S. in Speech Language Pathology and Audiology with a minor in Linguistics and a masters degree in Communication. My specialty was language development. I worked for a short time as a special ed teacher. Have not done this stuff in decades though.