Anders--
"Coming of age" socially usually refers to one or more ages at which a person obtains legal rights: 16, 18, and 21, for example... but religiously "coming of age" refers to the age at which a child becomes an adult in the eyes of their religion: religions may consider that age to be 13 (Judaism, I believe), 16 (UU Church), and 18 (some Wiccan covens).
Maize--
There is some kind of little ritual in the UU Church. My congregation has a mentorship program for high school students (I believe 16 years old) that basically teaches them about "being adults" and ends with a ceremony where they cross a symbolic bridge. It is not much, but it is something, and I'm willing to bet it is significant for the kids who go through it.