Steps:
1. Go to Admin> Users>Click on Edit icon of the admin user
2. Select Customer Name from drop-down list and observe the Is Admin checkbox
3. Now, You can see that Is Admin checkbox will be disabled by the user is Admin user.
Note: Is admin checkbox is not disabled for all users. Its only for the users, which select the customer name.
4. On setting the customer name as –Select–, Is Admin becomes enable again.