My guess on mix would be australian shepherd mix of some kind. Yes, there are other breeds that come in merle, but aussies are probably the most common and therefore the most likely to be running around making feral/unwanted pups. The only other "common" breeds that are frequently merle are collies and shelties, and her muzzle is too short and broad for me to suspect either of those breeds. She does look like she could be a catahoula mix, but those generally aren't common outside of the south.
As to her age, she is probably around 3 months old. She's at least 2 months (all of the baby teeth are present and they are all in by 8 weeks), and less than 4 months (at 4 months the first adult teeth, the two middle upper incisors, start coming in).
As to her age, she is probably around 3 months old. She's at least 2 months (all of the baby teeth are present and they are all in by 8 weeks), and less than 4 months (at 4 months the first adult teeth, the two middle upper incisors, start coming in).