As girls were not allowed to lead religious services -- and still aren't in Orthodox congregations -- the concept of the bat mitzvah is less rotted in religious tradition and ceremony. There were often parties held to celebrate a girls entrance into Jewish womanhood, often held in their twelfth year as it was believed (and still is by many people, both religious and secular) that girls mature faster than boys. But these had no real religious significance, and were instead cultural and familial ways to celebrate a girl's coming of age. Today, bar and bat mitzvah's are conducted in much the same way in many reform and conservative congregations.
Another change that has occurred in the modern concept of the bar mitzvah beyond the addition of a major ceremony to commemorate this event, according to Judaism 101 (jewfaq.org), is the level of involvement in the service that many bar and bat mitzvahs have. The coming-of-age that the ceremony symbolizes happens automatically at a certain age (thirteen and twelve for, respectively, boy and girls). At this age, Jewish youths become responsible for obeying all the commandments of Judaism. Though following the laws of the Sabbath and of keeping kosher are highly encouraged and usually followed throughout childhood, it is at this age that children are considered morally responsible. It is only in the last century that the idea of making this responsibility to God's laws manifest on a special day set aside for the event has come into vogue, leading to the religious leadership role many bar and bat mitzvah's take on.
At modern bar and bat mitzvahs in non-Orthodox congregations (and some Orthodox congregations, though it would be limited to bar mitzvahs), it is considered customary for the bar/bat mitzvah to lead the entire religious service on a Sabbath morning. This includes greeting the congregation, leading all of the call and response prayers, opening the ark where the Torah is kept and carrying it through the congregation, and not only saying one set of the opening and closing prayers for reading the Torah (there are seven readings and seven recitations of the prayers every Saturday), but reading either one section or all seven sections of that week's reading from the Torah itself. Bar mitzvahs take months of preparation; it is not unheard of for students to enroll in individual tutoring sessions for a full year before the big day comes. In such bar mitzvah's it is also often likely that the training is the first time the students have seriously studied their religion; most Orthodox bar mitzvahs are not required to study as much specifically for the event because they have been studying torah for the past three years, and praying all their lives.
Very little of the description of a modern bar mitzvah applies to the event I witnessed. Apparently, this bar mitzvah did even more than was required by giving a dvar torah, but he certainly did far less than what most modern bar mitzvahs are described as having to go trough. This did not diminish the palpable sense of excitement or achievement in the gathering of the congregation in the slightest, however. It was obvious that the essential quality of recognizing and celebrating this individual's acceptance into the adult religious community was present in spades, and according to the sources I encountered this is all that a bar mitzvah truly is.
There were some elements of the other trappings of a bar mitzvah in the ceremony. His aliyah (the recitation of the prayers for the Torah), for example, has been a standard (though not scripturally mandated) part of the bar mitzvah tradition for a very long time. The dvar torah is also a common touch. Modern bar mitzvah's also often include a speech about what it means to the individual to become a full adult member other religious community. Often these are full of thanks to family and friends, and sound basically like graduation speeches. I must say, I am somewhat relieved that such a speech was not delivered at this bar mitzvah.
The most direct similarities in the descriptions of bar mitzvahs that I read and the one I attended were the basic details of the Shabbat service itself. There were changes here, to, but the basic set up of the ark, bimah, and congregation was the same. The division of genders is only practiced in Orthodox congregations, but the division of congregation and Rabbi has become standard (though this, too, is a relatively recent development in the practice of Judaism).
The biggest similarity between the descriptions and my observations, though somewhat more abstract than the details of the service, was the purpose of the event and the feelings of the family, congregation, and the bar mitzvah boy himself. Everyone involved rejoiced at the occasion; the parents and family of the bar mitzvah had watched their son grow into a man capable of explaining parts of the religions most sacred text, the congregation had earned a new member, and certainly the bar mitzvah boy reveled in his new sense of freedom and responsibility that are attendant upon adulthood. Though this adulthood is recognized only in a religious context and not by society or by practice in the boy's family or even in the Jewish community, it is another signpost on the way to true adulthood. The pleasure that such an occasion brings does not require any study to recognize, but is etched on human faces in much the same way regardless of culture or religious…