Upsherin is not a universal Jewish practice observed across all communities; it is specifically a Hasidic and closely related Kabbalistic custom, absent from mainstream Sephardi practice and not treated as required even in much of non-Hasidic Ashkenazi Orthodoxy, which is why a three year old boy's first haircut looks like a significant milestone in some Jewish communities and passes unmarked in others. Its most commonly cited justification borrows an analogy from a Torah law that has nothing to do with children at all: Leviticus's rule that a newly planted fruit tree's produce is forbidden for its first three years and only becomes usable, eventually sacred, in the years that follow, a law some Kabbalistic sources read as a template for a child's early development, treating the first three years as a period before certain religious markers, cutting the hair among them, are appropriate.
The custom's best known public expression is not a private family event at all but a mass celebration held at the tomb of the second century sage Shimon bar Yochai in the town of Meron on the festival of Lag BaOmer, where families from across the Hasidic and wider Orthodox world bring three year old boys to receive their first haircut amid large crowds, music, and celebration, a single site and occasion that has done more than any other to make upsherin visible outside the specific communities that originated it.