Q. It seems to be a widely accepted custom in the orthodox community to wait for three years before giving a child his first haircut, but I've never heard a good explanation for this. Where did this custom originate? Does it apply to girls as well? Isn't there a prohibition against dressing or appearing like the opposite sex? Thank you for your help!
-Evan
This is a great question. The earliest mention of the concept of Upsherin
(Yiddish for first haircut of a boy) appears in the works of the Radvaz (Shut
Haradvaz II:608). There, the Radvaz was asked about what one should do if he
took on the Neder (and oath) to cut his son's hair at the Kever (gravesite) of
Samuel the Prophet and discovered he couldn't get to the Kever because it had
fallen into the hands of the Yishmaelim (Arabs). Apparently, in the times of the 
Radvaz this seemed to be a common practice, that male children received their
first haircuts at the Kever of Shmuel Hanovi. The hair was collected and weighed
and a corresponding amount of gold or currency was donated to the upkeep of the
Kever.
The aliyah (or ascent) to the Kever of the most famous Kabbalist, Rabbi Shimon
Bar Yochai (a common practice in both Chassidic and Sephardic cultures) seems to
be based upon the writings of Rav Chaim Vital (Shaar Hakavanos 87a). He notes
that before he began learning with the Ari he was informed that the Ari had
taken his son to the Kever of Rabban Shimon Bar Yochai and cut his hair before
he began learning and a big celebration ensued. However, it should be noted that
his son's age at the time was not specified. These sources only point to a
custom to cut hair of young children at the Kevarim of Tzaddikim and to make
donations to Tzedaka (charity) and some Seudah (festive meal) as a result.
If one examines the Shulchan Aruch (Code of Jewish Law) one will find no mention
of the Minhag of Upsherin. The grandson of the Tzemach Tzedek determined (Seder
Tefilla L'Maharid) that the story of the son of the Ari must have happened when
the boy was three and therefore the Minhag of Upsherin is to cut the boys' hair when they turn three. Some associate the age of three with the age when Avraham
first recognized Hashem (See Nitei Gavriel, Tigalachas HaYiladim 1:2).
Others find hints in the verse in Leviticus which appears in reference to the
Mitzora where the letter Gimmel is raised in the word "Vishkalach" (and he will
cut his hair) to be a reference to the age of three (Gimmel is the 3rd letter
of the Aleph Beit). (AteresYishuah Moadim, 33a). Still others associate the age
of the first haircut with the Mitzvah of Orlah, the restriction of benefiting
from the fruit of a tree within the first 3 years of its growth. When the child
turns three, in certain circles, he begins to attend the Yeshiva and to study
Torah, just like entering the fourth year in the life of a tree, he can now be
dedicated to Kodesh (holiness), and his parents can begin reaping the "fruit" of
his labor.
For any, and all of the reasons given above, it is apparent that there is really
no source for this minhag to be practiced by girls reaching the age of three.
Aaron Parry
