Each culture has its ages where milestones are reached. In the U.S., the major milestones are generally considered to be at 16, 18, and 21. These ages are important because they are significant legal milestones. At 16, you can drive. At 18, you can be drafted, vote, marry without parental consent, and be legally tried as an adult. At 21, you can drink alcohol. These ages do vary from state to state and have probably changed a bit since when I hit these ages, but they are largely the same in most places in the U.S.
These ages are also milestones from the viewpoint of how others perceive you. In my family, my mother loved to repeatedly say that there was absolutely no "dating" until age 16, and it still continues to be the age at which more conservative parents feel it's appropriate for their kids to start actively engaging in more formalized relationships with a significant other, though this is less and less the case as the age of the onset of puberty goes parenting notions become more liberal. Eighteen is the age at which you are expected to start taking financial responsibility for yourself and at 21, you should be comporting yourself as an adult to a great extent.
Age-based milestones are probably more diverse across cultures than many other artificially-assigned aspects. That is, while things like food and dress are directly related to the environment that a group of people live in, age-based notions are much more related to subjective concepts of maturity (though certainly not in all respects as the onset of puberty factors into all perceptions of adulthood). In Japan, there are milestone ages but there are fewer of them at younger ages and more of them at older ages compared to the U.S. The age at which one is considered an adult in Japan is 20 and all people who turned 20 in the previous year celebrated their coming of age on January 14 this year.
If you read any Japan blogs or news sites, you will have seen pictures relating to this and little snippets about what it all means. This particular holiday holds a good deal of interest for foreigners because it's the one day of the year when you see a large number of young, well-made-up women walking around in their finest kimono. It's a day when the "old" view of Japan as having a plethora of refined, fresh beauties in traditional dress mingles most vividly with the newer, more mundane version which tends only to appeal to the fetishistic appetites of gaijin when schoolgirl uniforms or cosplaying women come into view.
Young women walk the streets of Ikebukuro in their kimono, freezing their buns off on a chilly winter day with only a fur stole to comfort them.
For some Japanese women, this day is a rather complex one with a relatively unglamorous, but warm-hearted goal in mind. One of my students turned 20 in the previous year and she filled me in on the "behind the scenes" situation for a young woman approaching this day. Several months before the actual holiday, her parents took her to a hotel where her hair and make up were professionally done and a series of photographs of her in her adulthood day kimono were taken by a professional photographer.
The whole experience was tiring and unpleasant for her as it took a long time to prepare her and wrap her up in her kimono. The make-up was also quite itchy and the woman who prepped was a bit terse and pushy. In the end, she wasn't very happy with the pictures and believed that she never quite managed to pose as the photographer asked. She felt her chin looked bulgy when her head was tilted down and the photographer kept asking her to hold it differently, but she never quite managed. I can't say that I agreed with her regarding the photographs. I thought she looked beautiful.
The strange thing about this holiday is that women go through a lot to dress in traditional clothes and do their hair and make-up just right, but men just put on a suit. I'm not sure why men aren't running off to a professional and getting an artificial topknot glued to heir heads and forced to wear traditional men's formal kimono, but there appears to be no such expectation for them. Men get a much more comfortable and inexpensive day when they turn 20.
For the young adults celebrating this day, the biggest draw, and for some the only draw, is that it allows them to have a reunion with all of their friends from elementary school to high school. It's the one time when there's a good chance that people they haven't seen for a decade will come together and they can all catch up on each others lives. The more grandiose aspects like being allowed to vote and being considered a "member of society" aren't looming in the front of their minds. However, I'm guessing that having a day of reunion probably runs neck and neck in importance with being allowed to being legally allowed to drink in the minds of a good many 20-year-olds.