Oaksoul

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Oaksoul refers to a system member who looks like and/or acts like an adult, but is not an adult.

This can be for many reasons, such as, but not limited to, being in a median system, | age progression or having a metaphor-based internal age instead of a development-based internal age.

Oaksouled members may find it more helpful to be treated as their body's age, or they may prefer being treated as their apparent age. It's recommended to ask them what their preferences are, as it may not always be obvious.

Related Terms
A Sproutsoul is a member who looks like and/or acts like a teenager, but is not one.

A Seedsoul is a member who looks like and/or acts like a child, but is not one.

Lost adult may be considered a synonym.

Reference
https://the-mare-cognitum.tumblr.com/post/654655107267461120/flags