When do you consider someone an adult?

None of the above. Specific physical ages are important for legal reasons--to keep the playing field even--but having attained a certain age does not make one an adult. The last answer, when a person is mature and independent, is only mediocre. For example, my grandmother requires assisted living or a nursing home. Do we strip her of her adult status, now that she is no longer independent? I believe the single most defining characteristic of an adult is the willingness and ability to take responsibility for one's actions. The actions and responsibilities will change as a person grows older, but the capacity to deal with them as an adult continues all his or her life, for most people.

I thinks when he or she is mature(physically and mentally) normally around 18 and above.

Age is never a reliable way of determining if one is or isn't an adult. I've known a 16 year old who was far more responsible than a 50 year old man I converse with at the bar.