When you enter Xbox Insider Hub, it says that you need to be 18 to use it.

When I load the insider hub on my windows 10 and Xbox 1 it says that I am not 18 and so it wont allow me to use it but I am over 18.

That does not work. He can't access Xbox Insider Hub because he's not 18. It says this when he tries to access it and doesn't allow him in.

Okay, I get it. But before I was making the post, I saw someone else's post with the same question (that thread is now locked, unfortunately). It says that the user is over 18 years old yet they cannot access the Xbox Insider Hub because it says that they're "under 18". A reply said that the Xbox Insider Hub checks the age you originally created your account at and not the age you're currently at.

Hello Sífiso how are you?
My name is Ezequiel I'm an Independent Consultant. In order for you to have the betas you need to be part of Xbox Insiders, to sign up for the program it's very simple. Becoming an Xbox Insider is as easy as downloading the Xbox Insider Hub app on your Xbox or Windows device.
To get started with an Xbox console:
Go to Store from Home or select Store from the dashboard.
Select Search.
Enter Insider in the box, then select Xbox Insider Bundle.
Select Get or Install. This will install the Xbox Insider Hub and Report a Problem app.
To start using a Windows device:
Click the Start button , type store, then select Microsoft Store.
Select Search, enter insider in the box, then select Xbox Insider Hub.
Select Get or Install.
For more information about the Xbox Insider Hub and how to get it on your Windows device, go to:

https://support.xbox.com/help/account-profile/manage-account/xbox-insider-hub

If you have any questions, please contact us again.

Just look for the Xbox Insider Hub on your console and download it. You can do this on PC as well if you like. Once done you can enter one of the preview rings. The Insider Hub will give more information on what happens when you join.