In the Jewish faith, you become an adult when you have your bar or bat mitzvah, usually at age 13. You drink wine, read from the Torah, celebrate with all your family and friends, and are recognized as an adult in the community.
I have another idea about coming of age as a Jew. I'm pretty sure the real test is when you make your first brisket for Passover (am I right, ladies?). A big part of Passover is the food, and as anyone who has prepared a seder can tell you, it's pretty stressful. This year, I decided to take on the enormous task of preparing seder for my boyfriend's entire family, ten people in all. Go big or go home, right? Keep in mind, Boyfriend and co. are Catholic. His mom offered to make veggies, and his sister-in-law handled hard boiling the eggs for me, but for the most part, I was on my own.