Since we do have a rule against creating multiple threads on the same topic, there will be times it is good to just reply on an existing thread instead of creating a new one. BUT, it does get VERY confusing for members, and I think that anytime a thread that is more than a month old gets bumped (or past the first 2 pages on GA), you should have a solid and good reason.
Don't bring up an old thread just to post, "cool" or "bump" or "like that." Comments should be of substance.
Also, since the original participants are likely to be gone after a certain point, it is especially pointless to offer advice on an old question.
With your example on the dogs, unless there was particularly interesting information in the old thread, I wouldn't pull up something that was written in 2008, for the simple fact that the posters are almost always different than the current active ones, and it is confusing to see the old and gone names. Maybe 6 months to a 1 year for that sort of thing?
Basically, it is a matter of manners and judgment.
If you go overboard on it, our members will most certainly tell you 
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Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).