You cannot unless they do it.
Apple used to severely restrict changing the age on a child account to a non-child age. Earlier versions of the article: Update the birthday associated with an Apple Account - Apple Support said this had to be done by Apple itself, or even just simply wait until the child aged to a non-child age with no option to change the age any other way.
Now Apple appears to have relaxed the restrictions and Apple’s support article now simply says, “You can edit a child’s birthday by signing in to their account on the web.” Since child accounts can only exist as part of a Family Sharing group, the Family Organizer has to change the age. The article: If you change an Apple Account to a child account - Apple Support includes a special scenario “If your account is accidentally converted to a child account”.
If you do not find changing the age on the account as simple as the articles now imply, I suggest the Family Sharing Organizer contact Apple about this. This page has telephone numbers for contacting Apple for support and service ➔ Contact Apple Support - Apple Support