"The Deadly Years"
Where were you on December 8, 1967?  On that day, Steven H. Scheuer reviewed “The Deadly Years” for his syndicated column, TV Key Previews, writing:
Another imaginative tale that is well worth watching tonight. It involves a strange visit to a planet which injects all who land on it with a galloping case of old age and death. Everyone that is except for one person. Although Captain Kirk, Mr. Spock and McCoy are all smitten with an advanced state of old age and infirmities, it is Chekov, who also landed on the planet, who escapes. The enigma of Chekov’s continuing good health and youth finally lead to a discovery of a cure but not before a number of people have died of hoary old age and the Captain himself is removed from command. An intriguing episode.
Judy Crosby reviewed it her syndicated column, TV SCOUT. She said: 
A good imaginative script that is never over-played or over-costumed, as it might have been and this makes Star Trek a good bet tonight. It concerns a planet that has a strange ability to increase the aging process greatly. In fact, everyone on an Enterprise landing party, except Chekov, finds themselves rapidly approaching senility. And as he returns to the spaceship Captain Kirk find himself one of the victims. His judgment becomes so rattled that he is relieved of command by a passenger on the spaceship while the additional crew strives to find a cure as the Enterprise reels through space.
A.C.Nielsen counted the noses, and there were more noses (and eyes) pointed toward NBC that night than what we have been led to believe for more than four decades. Here are the numbers:  

RATINGS / Nielsen National report for Friday, December 8, 1967:
8:30 to 9 p.m., 61.1% U.S. TVs in use.    Share:        Households watching:
    
NBC:    Star Trek (first half)                       28.6%             12,770,000
ABC:    Hondo    (first half)                        21.8%               9,800,000    
CBS:    Gomer Pyle, U.S.M.C.                 43.6%             16,180,000
Independent stations:                                  6.0%                No data

9 to 9:30 p.m., 61.8% U.S. TVs in use.    Share:         Households watching:

NBC:    Star Trek (second half)                 28.1%             No data
ABC:    Hondo (second half)                     23.6%             No data
CBS:    Friday Night Movie                     36.8%             No data
Independent stations:                               11.5%              No data
See all the ratings to all the episodes and so much more, in the Saturn-award winning book series, These Are the Voyages: TOS.   You can get your copy here, signed by authors Marc Cushman, and for the best price anywhere, by clicking below.   
* While some of the images in the excepts are shown in color, all images in the book are in black and white.