system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are healthcare plans restricted to family members? And why do after a certain age even those family members are kicked off?
gpt
Because its based around who is dependent on you. 

If you are a husband, you help take care of your wife and take care of your kids.

You don't take care of your best friend Jim or your grown son or your brother.

Except in special circumstances. Many plans use the term "dependent" instead of "child" and can be altered if that dependent is incapable of taking care of them self (such as in a child with cerebral palsy or an autistic person, etc)

The idea behind it is that your plan takes care of those that are dependent on you. Not grown people who are able to care for themselves.