Apple's definition of adults in many cases is 13 or older. Are they 13 or older?
You can set up ScreenTime on any device. You may have to do this on the device itself (never done it myself). Keep the passcode to yourself so prevent others from changing it.
Set up Screen Time for a family member on iPhone - Apple Support
Use parental controls on your child's iPhone and iPad - Apple Support
If they are not 13 or older and they have the wrong age set, you can change the age to child. From the support article: Update the birth date associated with your Apple Account - Apple Support
To change a birth date to any age below 13 (age varies by country), the person must be a member of a Family Sharing group [ Set up Family Sharing - Apple Support ], and the family Organizer must approve the change. After the birth date is changed to any age below 13, it can't be altered except by contacting Apple Support --> Contact - Official Apple Support
1. Follow the instructions in the article to update the birth date on an Apple mobile device or on the web.
2. Follow the onscreen steps to send an email request to your family organizer.
3. Your family organizer will get an email with instructions to approve the request.
Read the document for complete details.
Also of interest: Approve what kids buy with Ask to Buy - Apple Support