at what age should someone be considered an adult?

this may be a stupid question to some but at what age do you think someone should be considered an adult? i know that the legal age is 18 but in your personal opinion do you think it should be a higher age or maybe lower? just curious. i recently read a manga that made me think about if 18 should really be the age when someone is called an adult. never thought about it till now tbh. (when someone has life experience or when their brain stops developing should they be considered an adult? or 18)