I was able to change the birthdate on my currently 13 year old daughter (will be 14 in a few days) by making her a year older (when I selected her ACTUAL birth year and tried to save it, I received an error, which leads me to believe that her correct birthdate was set up in the first place). Once I made her 14, the Ask to Buy option appeared in the Manage Family section...
Progress, I thought... When I clicked on "Ask to Buy", I received the following message:
"XXXX is sharing purchases from an account also used by another family member. Ask to Buy can be enabled only on accounts that are not shared."
We all shared my account previously and I'm in the process of unlinking them... So maybe this will fix itself when I get through all the devices.