When do people's circadian rhythms tend to change or is it different for everyone?

There is no set age when people's circadian rhythms change. Just like a lot of things in life, this change depends on the person. Two main factors that influence your circadian rhythms are your environment and genetics. If two teenagers the same age have different environmental factors, for example light, their sleeping schedules can be completely different. One might be a "night person" and the other a "morning person" which means their circadian rhythms won't change from adolescence to adulthood at the same time, or even in the same way. However, even though there isn't a specific age or date when your circadian rhythm will change, their are ways you can change your sleep-wake cycle yourself.

Source:
http://www.washingtonpost.com/wp-dyn/content/article/2010/06/28/AR2010062803820.html