This is difficult to find out what makes an adult, an adult. It is not clearly related to age, or necessarily to maturity...I know plenty of 14 year olds which are more mature than many 30 year olds I know.
If you were assigned to perform research on the subject - what variables would you investigate? What makes someone ‘mature' or ‘able to take care of them mentally and physically'? How would you measure such concepts?