I am an adult but it says i'm a child?

No, my account is not a child's on a family account. It says that I am an adult on my family's account, not a child.

Ok I now have managed to make a new adult account but every time I'm trying to change it from the child to the adult account it says, Sign up not available.

There are three accounts in total, two adult, one child. The info I gave previously was from my adult account.

I'm just checking his child and our adult statuses.

My account is Adult and his is Custom (Child setting interfered with his gaming).

I can't remember 100% but I am sure whoever set up the child account can change it. Like if you made an adult account on the xbox before making the child account that adult account can change wether the child account is a child account or and adult one.
I will just test this out myself now and post how it went

When your account was created, it must have been a child account. Until the original birth date that was used when the account was created, reaches the age of 18. Your account will remain a child account until that date. Changing the settings of the account
to Adult settings, does not make the account an adult account. Nor does changing the birth date after the fact. Again, you'll have to wait until that account reaches the age of 18 based on the original birth date of the account.