Miri

TOS Season 1 Episode 8
Air Date: October 27th, 1966

The enterprise enter into a solar system that perfectly matches Sol. They 3rd planet from the sun is a planet that perfectly resembles earth. Land mass, oceans, even when they beam down the buildings are the same. Spock estimates that it is mid 1960s architecture (convenient). Turns out the episode has nothing to do with any of this and is just a red herring.

The actual story is based around the people that are left on the planet. McCoy gets attacked by a teenager with scar tissue all over his body. When Kirk stuns him, he ends up dying. After some research they find out that hundreds of years ago the people were researching a way to prevent aging and accidentally released a virus that kills anyone who hasn't gotten to a certain point in puberty. For the children, the virus does what they were planning it to do, it prolongs their life allowing them to age very slowly.

The question the episode tries to tackle is, does age have to do with maturity. The children on this planet are 300 years old, but they haven't gone through puberty. They are smart, but they still act like children. Eventually bones finds a vaccine for the virus and risks injecting himself to see if it works, which it does.