Hello
I am sorry, just to clarify - this is a personal account you are using for business purpose, meaning it is a free account you are setting up, right?
If so, please use the guided instruction here: https://support.microsoft.com/en-us/account-billing/how-to-change-a-birth-date-on-a-microsoft-account-837badbc-999e-54d2-2617-d19206b9540a
Expand the section "I am an adult, but Microsoft thinks I am a child", and start from there.