Can't join the Forza horizon 4 club

This means your Xbox account is a child account or you have a suspension preventing you from joining clubs.

Xbox account age can not be changed as it is permanently connected to the first date of birth entered when creating your Microsoft account. This can't be changed even by Xbox support. Your account will be promoted to an adult account when that original birth date reaches 18 (19 in Korea). You can change your Microsoft account age but that doesn't change your Xbox account age.

Please first see this page to see if you have any suspensions or if it says "you have a perfect record" https://enforcement.xbox.com/Enforcement/ShowEnforcements

If you have a perfect record with no suspensions then you will have to have an adult account add you to a family and change those settings for you.

Have an adult add you to a family group here and then that adult account can change those settings for you on the privacy settings page.

https://support.microsoft.com/en-us/help/12417/microsoft-account-add-members-to-family

If your Microsoft account is an adult account you may be able to access that page but any changes will not take effect for real if your xbox account is a child account.

If both are a child/teen account that page will take you to a page about adding a child account to a family.

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On your adult account go here https://account.xbox.com/Settings and click on the child's account and allow the option on the first page called "Share stuff people make (Xbox 360: Member Content)" to everyone (must be everyone). Also, on the first page, "Others can communicate with voice, text, or invites" to Friends or Everybody. And on the xbox one/windows 10 tab the second option down, "Join multiplayer games", enable that. Also enable joining clubs. And then log out and in on the account in question on the console/device and that account can play online and join clubs.

Your Microsoft account can be adult while your Xbox account can be child or teen.

If that's the case you can see that settings page but any changes will not take effect and some may say allowed when they are actually not.

Please contact xbox support for more help on this issue: http://support.xbox.com/contact-us