2 things to note:
- The age set for your Apple ID has no bearing on what apps you can download. Restricting app downloads is done in Screen Time settings.
- If it is set to under 13, only Apple Support can change it. click here ➜ https://getsupport.apple.com
If you have Screen Time settings limiting the Apps you can download, you can go to Settings ➜ Screen Time ➜ Content & Privacy Restrictions ➜ Store, Web, Siri & Game Center ➜ Apps, and set it to whatever age you want.
You will need the Screen Time passcode if one is set to access these settings.