Hey there, Yen Voon
I have an active Microsoft 365 Family subscription, but I am the only user of the subscription.
The Microsoft Family and Microsoft 365 Family subscriptions are two separate products. You have people inside your Microsoft family that do not show in the Microsoft 365 subscription. Alternatively, you can people who are using the Microsoft 365 subscription but not in the Microsoft family.
Correct the date of birth on the account.
As soon as you enter a date of birth that puts you as a child in your region, the date of birth field is locked and can't be changed. This is to help prevent children from bypassing family restrictions by changing the date of birth so they can leave the family.
Remove the incorrect child-account classification.
The Microsoft external staff, or Microsoft employees who participate in the forums, do not have a way to edit Microsoft accounts. This control is limited to a group of Microsoft staff who are able to do this. With this being said, I am unsure if they will do this. However, you would need to reach out to Microsoft support directly using this link: https://support.microsoft.com/contactUs
Please sign into your Microsoft account and select expand the list of products. Then click onto the link that says "I can't see the product I need help with"
This will allow you to create a support request to the support team. Like I said, I do not think they can change your date of birth, or be able to raise this with a team that can help. However, they will be able to explore what options they have available to them.
Remove any Family Safety restrictions associated with the incorrect age.
You mentioned that there is a family organiser in the Microsoft Family. The Family organiser should have an email address connected to their account, as any account added to Microsoft family should be a separate Microsoft account.
Please can you sign into your Microsoft account here: https://outlook.live.com/groups
Once you have signed into your account, please can you click onto the people icon on the your family card

This will display a list of all your members of your family. Seen as you have two people in your family, you and the organiser, you should be abele to see who the organiser is and their email address.
If this is your other Microsoft account, you can sign into that account and manage your privacy and family settings. If this is someone else's account inside your household, please ask them to manage your family and privacy settings.
They will need to sign into the Microsoft family site here: https://account.microsoft.com/family/home
They can click onto your name, and manage your family settings from there. For more information, please see this article here: https://support.microsoft.com/en-US/family-safety/managing-parental-consent
Hope this helps,
Nathan