Create an enumeration with suitable entries: Kid, Teenager, Adult...
Add an attribute to your user object set to this attribute. Add a Before Commit microflow to your User object to set this attribute from your Age field.
Add the same attribute to the objects (like films?) that you wish to restrict.
Use Custom microflows to open/display your (film?) objects to check the attribute value for the currentUser vs. the object being opened, and either display a message or open the object as appropriate.