If my child isn't an adult?

My parents are 54 and 57 and all my children went to the college. I was really boring,I didn't wqnt to wait and of coirse I dindn't want to kill parents,so because they culdn't have another child,I adopted one. He is so beautiful,I love him and I want to start new generation with him. But he is only three and parents probably will die in few years. Can I choose him when they die,or has him to be 18 to heredit the house?

All children who aren't yet 18 at the death of the parents will jump ahead in age and will be adults.