What should be the ideal age difference between an older biological son and an adopted daughter?
There is no ideal age that applies to every family. If the parents are ready for another child, it's ideal, whether the older child is six months or sixteen.
Whatever the parents decide. My parents adopted my brother, then a little over a year later put their names back on the list, thinking it would take awhile to get another child. I was placed with them nine months later. : ) We are 2 years and five months apart in age.
Pascal the Gambler
There is no ideal difference.
I wouldn't of thought it mattered.
that's up to the parents. and they need to be around to "parent" the children.
There is no ideal age difference, though in my experience a larger age gap between siblings is nice since with the older one being more independent, it's easier to put sufficient focus on the younger child.