You asked about the difference between a person’s carnal life and spiritual life. That’s the question I answered. The assumption which is leading you astray is that being born again produces immediate tangible fruits, and that once born again those tangible fruits persist continuously for the rest of one’s life.
As to what cannot happen as an infant, I guess you’re ignoring the case of John the Baptist.
Good point about expecting immediate tangible fruits, especially for infants

Though, I’m sure you and others might cite verses pertaining to what is expected of those past the age of reason:
Mt 3:8 “Therefore bear fruit in keeping with repentance;”
Mt 7:16 “You will know them by their fruits. Grapes are not gathered from thorn bushes nor figs from thistles, are they?”
Mt 7:19 “Every tree that does not bear good fruit is cut down and thrown into the fire.”
Perhaps some may see Jn 15:2 as pertinent to infant baptism and what to expect as a sign of truly being born again once the child has reached the age of reason:
Jn 15:2 ""Every branch in Me that does not bear fruit, He takes away; and every branch that bears fruit, He prunes it so that it may bear more fruit.
Do Catholic parents know which children will fall away after infant baptism? Unless they have a similar experience as Zacharias, with an angle telling him Lk 1:14-16 “You will have joy and gladness, and many will rejoice at his birth. For he will be great in the sight of the Lord; and he will drink no wine or liquor, and he will be filled with the Holy Spirit while yet in his mother’s womb. And he will turn many of the sons of Israel back to the Lord their God.”
While Catholics and Protestants may disagree on certain points of baptism and being born of the spirit, the question in the OP is still very pertinent to any soul. Those who respond defensively or aggressively might not yet understand the importance and relevance of the question. I was baptised as a Catholic, and was uncomfortable when asked if I knew I was born again and if my words and actions in everyday life bore witness to the fruit of God’s grace and my cooperation and response to His grace and mercy.
Michael
Phil 4:4-9