Immortality has been achieved, but the environment and economy are wrecked, so only a very small population can be supported. Very capable people are needed to slowly rebuild civilization, so everyone is tested and observed, and whoever doesn't measure up is killed and eaten or otherwise recycled.

The vast majority of people born will end up that way, usually in their teens and twenties, but sometimes earlier(when it's obvious) or later(if it looks like they might be late bloomers). Standards shift with the needs of the time.

Reproduction is manipulated in various ways, both to encourage desirable traits, and to maintain a large female majority to have a large pool of younger people to select from.

This might continue after the situation has greatly improved; Maybe the ones in charge like it that way, or worry about the dangers that change would bring. Killing all but the most useful people started as a sacrifice for the future; Now granting immunity from the culling is seen as something that needs to be strongly justified.