I have never fostered, but I was in foster care for a several years as a minor. Your experience will differ with each child, and the experiences each child will have had. I have known other foster children who had a very difficult time because their experience with foster care wasn't a positive one for them. But for myself, foster care bettered my life and gave me a bright future. Younger is definitely easier, though there are aspects that may be harder. In my experience, it is easier to care for a young child because usually their bad experience with possibly (for lack of a better term) unhealthy parents is more limited than with an older child. Keep in mind however, the younger the child, the harder they are to get placed with you. Many people want the younger children, because behaviorally wise they are easier in some ways. Its harder for a toddler to have a intense argument than a teenager.
As to your question about how hard it would be to care for a child and then watch them go back to a home where they were removed from? I think I can safely say that it could potentially be heartbreaking. I have seen rough cases where the parents were not able to have direct, unsupervised communication with their children who were in foster care and when the foster parents enforced this for the safety of the child, parents may become hostile and think of the foster parents in the wrong light. In that case, it may be difficult for you to continue to have any sort of relationship with the child afterwards, as I have seen happen with many foster families.
Either way, it is an amazing way to make a difference in a childs life. But it definitely isn't easy. The state can be difficult at times, the biological parents, and even the child (all depending on the circumstances) may cause stress. But dont, by any means, let that discourage you. There are so many children out there whose families are unable to parent them as they need, and those children really need people who are willing and eager to give them a safe and loving home. Good luck and I really that if you decide to become a foster parent, you have an amazing experience with it.