system
Given the following post, respond as if you're commenting on Reddit:
human
Is there a point when children become sentient? Like, is it just a sudden "I exist", or is it more of a gradual thing?
gpt
Once the can look into a mirror and realize that it isn't another child, but it is themselves. I'd say this is the closest realization to what you're trying to figure out. This is the self-awareness stage. 

Lev Vygotsky:


Stage 1 - Differentiation 
(from birth)
 Right from birth infants are able to differentiate the self from the non-self. A study using the infant rooting reflex found that infants rooted significantly less from self-stimulation, contrary to when the stimulation came from the experimenter.[27] 


Stage 2 - Situation 
(by 2 months)
 In addition to differentiation, infants at this stage can also situate themselves in relation to a model. In one experiment infants were able to imitate tongue orientation from an adult model.[28] Additionally, another sign of the differentiation is when infants bring themselves into contact with objects by reaching for them.[29] 


Stage 3 - Identification 
(by 2 years)
 At this stage the more common definition of "self-awareness" comes into play, where infants can identify themselves in a mirror through the "rouge test" as well as begin to use language to refer to themselves. 


Stage 4 – Permanence This stage occurs after infancy when children are aware that their sense of self continues to exist across both time and space. 


Stage 5 – Self-consciousness or meta-self-awareness This also occurs after infancy. This is the final stage when children can see themselves in 3rd person, or how they are perceived by others. 

I hope this helps!