[TOS] Season 2, Episode 12 (Netflix: S2 E12): The Deadly Years
The business about them getting old isn’t that great, and may even be a little insulting to old people. This episode mainly gets a 3 for its final scenes. They do a decent job building to the climax, which makes the ending a relief. It’s also another episode showcasing the command skills of Kirk.
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Read more at Memory Alpha
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Notes:

In the teaser some old people say they’re 29 and 27. So what? Some old people say they’re a little younger than they really are. That’s not a very suspenseful opening.

Kirk has an old friend that doesn’t serve much purpose in this episode other than to remind us that ladies really love Kirk

The landing party starts getting older! at a faster rate than normal, that is. Except for Chekov. He still seems pretty good and doing that exercise machine.

Kirk wants to send a message using code 2. The Romulans have broken code 2! Kirk is forgetting everything

The Commodore decides to go through the Neutral Zone to get to a star-base that can help Kirk and the rest. Romulans attack!

Kirk returns! He tells Uhura to send a code 2 message warning starfleet that’s he’s going to set off the corbomite, destroying everything in the area.
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