If when you created your Roblox account, you used a birthday that would make you under the age of 13, and you're still under the age of 13, there is unfortunately no way to manually alter your ...

Make sure that you always change your birthday, even if it’s a wrong one, to 13 years of age or older to be able to change it at a later date. How to change your age on Roblox account (PC desktop)?