At the age of 18 or 21, one would be considered an adult by law. You become an adult when you yourself can take decisions for yourself and you have the power to accept its consequence. When you are responsible for your actions, that is when you are an adult, and then it doesn’t matter whether you are 5 or 50. This is what I believe about adulthood.
When you are responsible and capable to accept the consequence of your actions, you are considered an adult. An adult doesn’t mean that you don’t make mistakes. An adult means when one does a mistake but then resolves it and tries not to do it again. It could take you many years to make you an adult. In the book THE GIVER, you become an adult at the age of 12. I believe that you don’t have an age to become an adult but your perspective of problems decides whether you are an adult or not.
You are never useless for a community. If you are young you get to learn from your elders, and if you are old you can teach. No community is too small to not make you a part.