My intuition is that the best chance of success is with children who are under the age of 3 when adopted.
That makes sense.
an infant adopted from an alcoholic, drug addict, bipolar or a schizophrenic is just as high a risk as adopted child at any age
genetics and prenatal abuse are important and sometimes the damage only shows up with age. A child's ability to master language and regulate their own behavior is a pretty important clue of many things
so the younger they are, the less you know what you're gonna get- like a box of chocolates
inherited traits affect your own bio kids too and sometimes for the worse