Charles,
I've heard several different definitions about what constitutes "historical fiction."
One writer specializing in historical mysteries defined historical as any era in which no one who was alive then is still alive.
Another writer differentiated between "historical" and mere "period" fiction, the latter being stories set in a particular past era but otherwise wholly fictional, while "historical" is fiction that's based on actual events. Hence, a Regency-era romance by Georgette Heyer would be mere "period," while the military novels of Patrick O'Brian, closely based on actual naval engagements during the Napoleonic Wars (which took place during the Regency) are truly "historical."
In an artcle I wrote about historical police procedurals, I defined historical as set far enough in the past that we can look at it with some historical perspective. Somewhat arbitrarily, I set 20 years, roughly a generation, as the cut-off.
By my definition, the 1970's are fair game for historical fiction.
BTW, the article's available on the 'Net. If anyone's interested, I can post a link.