When we were younger, we'd taken in a very special foster child. She'd ended up staying for ten years, ages 8-18 y.o. (It's a long story.) In short, this sweet young lady was devastated that she could not live with her mentally ill mother; her mother could not care for her and had given up 5 children.
Ages 8-18 are quite a challenge. Sure, a child can do more for themselves; yet, much more comes up in other ways. An older child is trying to sort things out, may be "acting out," is struggling to individuate and more.

We love her very much. We had our hands full with one. I could not imagine two or more, while giving each child all they may need. This young lady needed a lot, especially considering all she was going through with her mom/family constantly. She was truly devastated/traumatized that her mentally ill mom gave her and her siblings over to the state. This all caused her many psychological problems to resolve.

Lots to think about.

WC