Thank you for waiting. Before I give you some information, I do believe the very best thing to do with this little turtle would be to turn it over to a wildlife rehabilitator. A rehabber will know exactly what to do with it, and will return it to the wild when it is ready. In most states, it is illegal to keep wild box turtles because they are protected. Each one that is kept in captivity is one less in the breeding population. If you want to pursue that option, this link will take you to a directory of rehabbers in NM:
Dehydration is the biggest danger to a young turtle that has just left hibernation. A dehydrated turtle will not eat. Right now, I recommend giving the turtle a special soak in case its already somewhat dehydrated. Use half water and half unflavored Pedialyte (made for human infants). Soak the turtle for 20 to 30 minutes twice a day.
Pet stores are a very poor source of information on reptile care. While we should be able to count on them for information, it is most often wrong. You cannot judge a turtle's age by its size. That depends on temperatures, food available, and other conditions. It is, however, likely that this turtle is one of last year's late hatchlings. Turtles do not grow while hibernating, so size also doesn't tell you that it came out too early. You can read more about the natural habits here:http://www.desertusa.com/animals/box_turtle.html
The dried turtle food isn't really a good food. See if you can get some bloodworms from a pet store. They are sold for fish, and try that. Unfortunately, wild-caught babies seldom do well in captivity. They often slowly deteriorate. For that reason, the best decision would be to turn it over to a wildlife rehabber. Here is a site with detailed information on hatchling box turtle care:http://www.aboxturtle.com/box_turtle_hatchling_care.htm
If you want to try to raise the turtle, make sure it is warm enough, give it a couple of good soaks, then try feeding some bloodworms. If you have more questions, let me know by clicking on REPLY. I hope that whatever you decide, it will work out well.