In the sefer Minhagei Lita by Rabbi Menachem Mendel Poliakoff he writes as follows (pg. 69) “A new custom has sprung up among Jews today – to refrain from cutting a young boy’s hair until he is three years old. His parents cut his hair on the Lag B’omer following his third birthday, if possible, at the tomb of Rabbi Shimon ben Yochai in meron. In Yiddish this haircut is called Upsherren.
Before WWII, few people outside of Eretz Yisrael even knew there was a small group of Sfardi Jews who practiced this haircutting at the tomb of Rabbi Shimon ben Yochai on Lag B’omer. Except for a small group of chassidim, no one else adopted this extraordinary minhag.
After the state of Israel was established, observance of this haircutting became more widely known, and a new trend came into being. Although it was new to most Ashkenazi Jews, it was by no means a new idea. It was an old idolatrous practice mentioned in the Mishnah (Avodah Zara 8a). The gentiles of old used to cut boys’ hair at puberty as a pagan ritual. It was for this reason the sages forbade Jews from conducting business with these idol worshippers on that day. So what did today’s frum Jews do? They adopted this heathen custom and have become frum idol worshippers. This ceremony is absolutely a violation of halachah, as it is chukas hagoyim (a gentile custom), and should be discontinued. This is another example of the importance of heeding the principle of al titosh toras imecha, and not rushing headlong into adopting every newfangled idea.
According to my research, the Upsherren was an innovation introduced by the Kabbalists of Safed in Eretz Yisrael in about the 16th century. It was apparently not widespread, because before WWII, it was not well-known or much followed.”