Can I set an adult age to a child age in Apple ID?

No, that is not something you can do on your own, if you want to change the age to a child below 13 please Contact - Official Apple Support

If you want to change it to an child age older then 13, please follow these instructions

1. Go to Settings > [your name].
2. Tap Name, Phone Numbers, Email. You may need to enter your Apple ID password.
3. Tap Change Birthday or Birthday. 
4. Update your date of birth, then tap Done. If you're changing the date of birth for a child, find out what to do. 

No, that's cheating.