Not really. You can't really know unless you know when they hatch. If you post a pic we can tell if it is a young, not so young, or very old box turtle. But that's really about it.
Not without a reference. For instance, I have three juveniles all the same age. One is almost 4 inches and the other two are only a bit over 3 inches. Full grown they only get about 5 iches but they get rounder. Typically they can not fully close until around 3 - 4 years old. Size can not really be a determination of age but once they get the size yours appears to be I would say they are maybe between 6 -10 years old. No telling after that. Some would suggest you can tell roughly by the number of growth rings but that can be deceptive with captive animals as they tend to grow faster if not hibernated.