For children, the ideal is raising a dog from a puppy (with attendant development problems), but an older dog with a known past and one previous owner can be just fine. The big concern would be adopting a dog with unknown background. Many adopted dogs can be perfect angels for the first several months or longer, but a bad history can begin to show in strange ways once they are used to their new environment and begin to assert themselves. This is a concern for anyone, but more so with children. An adult environment is less concerning as long as the new owners are aware of the potential difficulties and are prepared to work with them.
Our current dog, Tonto, was supposed to be from a known background and we adopted with no concern. Sadly, it appears someone wasn't fully forthcoming. Tonto is wonderful dog, not vicious or aggressive, but clearly has issues which the previous owner could not have been ignorant of. He is ours and we love him and just hope the right environment and handling will eventually allow him to overcome his issues. (What issues? Try to picture a really nice dog with autism or possibly brain damage that was NOT a trait in his early years. Sometimes he acts perfectly normal, at other times (pretty much starting each day) it's like a switch was thrown and we have a fearful, anxious dog totally lacking in trust. Quite difficult for us and very sad. But there is no way we are giving him up.)
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