Can I use Default Accounts for setting up AOAGs 2014\2016
Can I use the default accounts which SQL Service provides by default


Yes you can use that. From Prerequisites and Recommendation for AG


  if the SQL Server service is running as a built-in account, such as Local 
  System, Local Service, or Network Service, or a nondomain account, you
  must use certificates for endpoint authentication, and the wizard will
  be unable to create a database mirroring endpoint on the server
  instance. In this case, we recommend that you create the database
  mirroring endpoints manually before you launch the wizard.


The advantage with using domain account is that they are more secured and when you configure availability groups using SSMS GUI the endpoints are created by the SSMS GUI and also granted the connect permission(. As you can note from above that when using built in account you need to create endpoints using certificates manually and grant connect permission.

You also have option of running SQL Server services with different domain account, if you do so you just need to make sure both the logins are created on remote machine in master database.

More over before changing I would suggest you to read Troubleshooting Always ON Configuration( see the accounts section)