Just wanted to throw our experience out there:
We adopted three of our children from foster care two years ago; our biological boys were 1 and 3, and before adopting our three boys we fostered an 8 year old girl for 8 months, hoping to adopt. I don't think, at the ages our biological boys were, disrupting the birth order would have been problematic, but the 8 year old had behaviors my husband and I weren't mature enough as parents to handle at the time, so she had to go to a different family.
A couple months later our three boys (biological brothers) were placed with us, with the intent to adopt (different states have different policies; in CA they rarely terminate parents' rights before finding an adoptive home, but other states seem to terminate rights sooner sometimes). So they still had visits for a month or two with their biological father, but then rights were terminated and those stopped.
Our oldest adopted son is six months older than our oldest biological son, and at the time they were placed with us their ages were 3, 3, 2, 1, and 1. We became pregnant with our daughter a few months after the boys were placed with us, and so when she was born our kids' ages were 4, 4, 3, 2, 2, and newborn. Really, the "roughest" time was when the boys were first placed with us; there's a bit of an adjustment time while they learn the ropes of a new family and we get acquainted with each other, plus all the new dynamics of the new relationships in the family. But now, with their ages being 6, 6, 5, 4, 4, and almost 2, the kids are quite the team, and we can't imagine life without them. They have different strengths and quirks and they play with each other all day long.
We are currently living in a state that allows people to adopt through the foster care system without needing to be certified as foster parents - you still go through a home study and certification process, but you can choose to only be eligible for legally free children. This limits your options to children who are, for whatever reason, not being adopted by their current foster family. Often behavioral or medical, but not always.
With this adoption, we will likely wind up disrupting the birth order again, and a bit more significantly, since our children are older this time around. In general, social workers do try to make placements work out - they don't want to have to move a child because something bad happened between children in a home - so (not surprisingly) we've been turned down on a lot of children we've submitted our home study for because the social worker is not willing to place the child in a home where they would be sharing a room with younger children.
All that is to say ... with this being your first exposure to the foster system, it is probably prudent to stick with an age you've already experienced. We learned that the hard way with the 8 year old we fostered. Now that our own children are a bit older, we're comfortable going a little bit older, since we have more experience as parents. It's not just a matter of hand-me-down clothes like a previous commenter mentioned. It's that children in foster care have often experienced levels of trauma we can't wrap our minds around. Young children - toddlers - will sometimes already be acting out sexually. Our adopted children have a half sister who has an STD, and she left that environment when she was 1.5 years old. Her adoptive parents had to wear gloves to change her diaper. Even young children from foster care can have issues - and probably will have issues of one kind or another - but the older the children are the more serious those issues can be.
Financially, states do compensate you for both fostering and adopting. Even if you move out of state, you will receive a monthly stipend check until the child turns 18. Plus there are adoption tax credits. Possibly not enough to entirely balance out the lifetime cost of raising a child, but it does help, especially if, like us, you end up in "crazy big family" territory. :-) But all the money they pay you wouldn't be enough to make you do it if your heart wasn't in it, and if your heart is in it, it wouldn't matter if they never paid you a penny, if that makes sense.
It isn't easy bringing in broken children, but it has been a huge blessing for our family. There are lots of resources about parenting children who have experienced trauma; Karyn Purvis is a well-known expert.
That's probably enough rambling, I hope that might have been helpful for someone. I don't log on here often, so unfortunately if anybody happens to have questions about our experience I may or may not see them, but I will try to remember to check this thread sometime next week just in case.