Hello, thank you for reaching out to Microsoft Q&A.
If Microsoft recognizes your account as a child account, it is possible that you might have entered your D.O.B incorrectly in your Microsoft account profile page.
Sign in with your Microsoft account on the link below and see if the D.O.B is correct:-
https://account.microsoft.com/profile
If D.O.B is incorrect, you will have to change it. However, only the family organizer has permission to change the D.O.B in your Microsoft account.
For reference, see the 'answered solution' on the thread below:-
I can't change my birth year in my account
Once you update your profile and your account is recognized as an adult, purchase the OneDrive Upgrade and check if it works.