We want to adopt my cousin

My fiance and I really want to adopt my second cousin who's Grandparents haven't been taking and have been allowing dangerous and emotionally scaring things to happen to him and his siblings. And as I know what he's been going through, as I've gone through it before, I thought I may be able to help him and show him the things he's more then likely thinking aren't true. As such we are also trying to get the three children to stay in the family. His younger sister is being taken by my mother, and is still in the works of happening. And his older brother, we are trying to get his mothers sister to take. All three are very good kids and we love them dearly. But what I was wondering, do both parents have to be above the age of twenty-one or can one just work out? As he's and older child entering his tweens, we know we could support him financially, with us. Both being very hard-working and responsible. I wanted to know if there is anything I am missing? Or any information I am missing, since laws vary from state to state.