I adopted a girl from the state. She started out living with me as a foster child. I suggest that fostering is a good way to find out how another child will fit into your family.
My daughter was 7 when she came to live with me. I didn't actually adopt her until she was 12. She was a special needs child with serious emotional issues. By putting off the adoption the state continued to pay for her therapy bills.
She was an at risk placement meaning that the state was working on terminating her birth mother's parental rights and so I didn't know for sure that I could adopt her. I knew that I wanted to after a few months. It did take 3 years before she was available for adoption. In the meantime I had the support of Children's Division workers as well as mental health specialists.
Not all older children are as difficult as my daughter was. When you apply with the state, if you decide to go that way, be honest about what you expect and feel that you're able to handle. Some require much more help than others.
I suggest that you talk with your state's office as well as private adoption agencies. The big advantage to going thru the state is that it costs nothing and if you start as a foster parent you get financial assistance.
You can adopt thru the state without fostering first. The actual cost of adoption is paid by the state. No out of pocket money except that normally needed to care for a child.
When going thru the state, you first fill out an application and take a several weeks course giving information and some training. Then you're assigned to an adoption worker who will then look for a child that matches your needs and theirs. The process does take several months.
When you talk with an adoption worker they will discuss with you what ages seem most likely to fit with your family. The needs of the child as well as your needs are taken into consideration. For example my daughter needed to be an only child. Sometimes it's best that they be the oldest or the youngest, depending on their life experiences.
Adoption is a challenging road. I bonded quickly with my first foster child and found the process worth the hassle and pain. There are rules, some of which seems to not make much sense, to follow. And if the child's birth parent's rights are in the process of being terminated or even already terminated there is the adjustment period for both your family and the child. This adjustment is very much influenced by what the child has experienced during the years prior to coming to live with you.
Call various agencies and the state. They will talk with you as long as you have questions. They will be able to give you an idea of what it's like with their agency. Each one will have some differences.
There are open and closed adoptions. A friend had an open adoption with their first child. They sent pictures every year to the birth mother. Another friend also had an open adoption and she and her family are closely involved with the adoptive family. The grandparents babysit. The birth mother and her parents are invited to family events. It's like an extended family.
I had a closed adoption because the birth mother was mentally ill and disruptive of my relationship with my daughter. However, I maintained contact with other relatives so that my daughter would have the security of knowing where she came from and have an extended family. This worked out well for all of us.