We Never Know
No Slack
That would depend on how you are defining "consciousness" and "one."
If you mean the capacity to have thoughts, emotions, feelings, personal identity, the perception of self and others, i.e. sentience, or what I view as the ego-self, then "one" would develop "consciousness" at between 24 to 28 weeks of gestation.
However, if you mean pure awareness, in my understanding, this "consciousness" is eternal. "One" experiences a life as a fetus/baby/child/adult.
Aware and one as itself.