The Deforms introduced it for 16 year olds as a way to keep them in Hebrew School longer.
https://en.wikipedia.org/wiki/Confirmation#Judaism Judaism
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Jewish confirmation c. 1900
All mainstream denominations of Judaism have a ceremony known as Bar Mitzvah or Bat Mitzvah, which occurs when Jewish children reach 13 years (boys), 12–13 years (girls); at this time they become responsible for their observance of Judaism's religious obligations. Prior to this, the child's parents hold the responsibility for the child's adherence to Jewish law and tradition. After this age, children bear their own responsibility for Jewish ritual law, tradition, and ethics and are privileged to participate in all areas of Jewish community life.
In the late 1800s Reform Judaism developed a separate ceremony, called confirmation, loosely modeled on Christian Confirmation ceremonies. This occurred because, at the time, Reform Jews believed that it was inappropriate for Bar/bat mitzvah age children to be considered mature enough to understand what it means to be religious. It was held that children of this age were not responsible enough to understand what it means to observe religious practices. As such, the reform rite of confirmation was originally a replacement for the Bar/Bat mitzvah ceremony, held at age 16. In later decades, the Reform movement modified this view, and now much of Reform Judaism in the USA encourages children to celebrate becoming Bar/Bat mitzvah at the traditional age, and then has the confirmation at the later age as a sign of a more advanced completion of their Jewish studies.
Today, many Reform Jewish congregations hold confirmation ceremonies as a way of marking the biblical festival of Shavuot and the decision of young adults to embrace Jewish study in their lives and reaffirm their commitment to the Covenant. The confirmands represent "the first fruits of each year's harvest. They represent the hope and promise of tomorrow."[69] Confirmation is typically held in tenth grade after a year of study, but some synagogues celebrate it in other years of high school.
Confirmation, in the context of Reform Judaism, was mentioned officially for the first time in an ordinance issued by the Jewish consistory of the kingdom of Westphalia at Cassel in 1810. There it was made the duty of the rabbi "to prepare the young for confirmation, and personally to conduct the ceremony." At first only boys were confirmed, on the Sabbath ("Shabbat") that they celebrated becoming Bar Mitzvah; the ceremony was performed at the home or in the schoolroom. In Berlin, Jewish girls were confirmed for the first time in 1817, in Hamburg in 1818.
Confirmation was at first excluded from the synagogue, because, like every innovation, it met with stern opposition from more traditional rabbis. Gradually, however, it found more favor; Hebrew school classes were confirmed together, and confirmation gradually became a solemn and celebration at the synagogue. In 1822 the first class of boys and girls was confirmed at the Hamburg Temple, and in 1831 Rabbi Samuel Egers, a prominent traditional rabbi of his time, began to confirm boys and girls at the synagogue of Brunswick. While in the beginning some Shabbat, frequently during Chanukah or Passover, was selected for confirmation, it became increasingly customary, following the example of Egers, to perform the ceremony during the biblical festival of Shavuot ("Feast of Weeks"). It was felt that Shavuot was well suited for the rite, as it celebrated the occasion when the Israelites on Mount Sinai declared their intention to accept the yoke of God's Law, so those of every new generation should follow the ancient example and declare their willingness to be faithful to the Sinaitic covenant transmitted by their ancestors.
Confirmation was introduced in Denmark as early as 1817, in Hamburg 1818, and in Hessen and Saxony in 1835. The Prussian government, which showed itself hostile to the Reform movement, prohibited it as late as 1836, as did Bavaria as late as 1838. It soon made its way, however, into all progressive congregations of Germany. In 1841 it was introduced in France, first in Bordeaux and Marseilles, then in Strasburg and Paris, under the name "initiation religieuse". The first Israelitish synod in 1869 at Leipsic adopted a report on religious education, the 13th section of which contains an elaborate opinion on confirmation, recommending the same to all Jewish congregations. In America the annual confirmation of boys and girls was first resolved upon by the congregation of Temple Emanu-El of New York in 1847. The ceremony soon gained so firm a foothold in America that soon there was no progressive Jewish congregation in which it did not occur during Shavuot.
Orthodox Judaism criticized the Reform movement for introducing confirmation,[citation needed] as the ceremony had no roots in rabbinic Judaism. When Conservative Judaism began to develop as a distinct movement, it too generally rejected confirmation as either unnecessary, or as a non-Jewish innovation.[citation needed] Today, nearly all Reform communities have returned to individual Bar Mitzvah at 13 and 12 or 13 for the Bat Mitzvah ceremonies[citation needed], but the communal confirmation ceremony is still popular.[citation needed]