At what age is Metronidazole safe to use?

Depends on why you are giving it........not puppies nursing for sure.

Check with your vet. I have used it on nursing puppies without any problems then or now.

I have used it on 7 week old puppies who had a case of the squirts, under direction from my vet.

Any age is safe as soon as you do the right doses. You should not need to use it in puppies younger than 2 weeks. Check with your vet.

the puppy is 4 months old.