I just want to know how old you have to be for it to be legal
Ms. Muirhead is correct but normally courts don't allow someone to take custody of other people's children. I don't know what your exact situation is and maybe you shouldn't say in this forum but I recommend that you contact a family lawyer.
In Nevada, if the parents are deceased, inappropriate, unfit, or unwilling to care for the children, then the court can either appoint someone else as the guardian or the custodian of the children or if they find it in the children's best interests.
It's not "normal", but it does happen.