In this world people get tf's baked into their very DNA. They'll grow up like normal and then one day they start to change. How exactly they change and how long the changes last vary so much they are all considered to be random. For instance some changes happen instantly, but others seem to last so long they last up until they die.
There are institutions that attempt to help, figure out what triggers these changes, but considering the variance and complexity of the genetic code there has been very little progress made on figuring out these transformations. However some correlations and rules of thumb had been made since old times. First most people transform for the first time during puberty some theorize this has to do with the introduction of hormones or something, but no one's really sure. Second is that while rare people have been known to have second, third, and sometimes fourth transformations. Finally while some people call it paranoia or seeing trends that aren't really there people have made a third rule stating that god has a sick sense of humor as semi-often people get just the right transformations at the most ironic of times.