Gosh we've never even thought how old we'll be when our kids move out, I think there's probably a good chance they won't all be out the door right on 18. We plan to start doing foster care though in our 40s when our kids are a bit older.
The finances of what they need when they get older def come into it. My fella is only 2 and I already feel we need to start saving up for braces, something towards a car for him and his higher education. Times that by 3 or 4 kids and that would def add up!