I just flew with northwest 2th of October. I was flying from florida to canada with stops in detroit and minneapolus so I went thru security many times. I needed a health cert. no one ever looked at it. At ticketing they look and your ticket will have a pet addition to it. once leaving ticketing you go thru security where the carrier is x-rayed and the dog/dogs must be removed and carried thru by you then they go in the carrier again. when boarding they are only concerned that you have the pet addition to the ticket and that the dogs are cute.......which I assume they will be
They are too busy at the security to put an age on your dog and they wouldn't care there anyway . and once they are in the carriers no one can tell if they are 6 months or 6 years.
Hope that helps
Rhonda