I think when I got a responsibility about my life I had accepted I'm adult. = I think people had accepted me as an adult the moment I got a responsibilty in life.
sometimes I'm thinking about my feature and making a plan. = Sometimes, I think about my future and make a plan.
at that time I feel that I'm adult now,, but it depend on individual person,, = At that time, I feel "I'm already adult".  But considering oneself an adult depends on every person.

Comment:

~ Thank you for doing your homework Mr. Jae. We value your enthusiasm for learning! Stay enthusiastic! Study harder! :)