A better stack for your Firebase Rules - Compiled Rules

rules.json

{
  "rules": {
    "users": {
      ".read": "auth != null && auth.uid == 'SERVER'",
      "$userId": {
        ".validate": "newData.hasChildren(['displayName', 'age'])",
        "displayName": {
          ".validate": "newData.isString()"
        },
        "age": {
          ".validate": "newData.isNumber()"
        },
        "$other": {
          ".validate": "false"
        },
        ".read": "auth != null && auth.uid == $userId",
        ".write": "auth != null && auth.uid == $userId || auth != null && auth.uid == 'SERVER'"
      }
    }
  }
}