The topic about when one really develops to become an older is really depending on how one interprets it. Different areas all over the world have steps and events that serve as right of passage from child decades to maturity. This however may only act as meaning. Does being an older really depend on a ceremony?
My personal Recommended Reading view about when one becomes an older is when they are able to make powerful decisions and choices that positively effect their lifestyles. Not many are able to do this. Being an older to me comes with several obligations, an older should be in a excellent place to deal with them all or at least have an idea on how to do so.