I think I know what you mean. Mine will be gone in less than four years and I often think about possibly adopting some kids later on.
I don't know the answer, but I know I still have lots of thinking to do. As inBoil said, you have to figure out what you're looking for. Is it mentoring, loving, or just having something to occupy time? How much time and effort are you willing to give? Will you be willing to give them up, if and when that might be required?
It ain't easy. That's the only thing I know for sure.