Just a quick question, I was watching the pokemon worlds senior division championships and the competitors looked older than 15, are the age divisions different at worlds?
So, it goes by the year of birth. Because of that, players won't move between divisions if they age up into a new division within that given season. The age divisions for the 2019 season are:
Junior Division: Born in 2008 or later
Senior Division: Born in 2004, 2005, 2006, or 2007