AGM's have a relatively low self-discharge of 1% to 3% per month.
So, a December 2012 battery would be about 8 months old and have discharged 8% - 24%. But, I believe the rate will be higher unless it was a cool storage room. You should consult the manufacture owners manual or err on the side of caution.
Are they willing to test it for you or sell it to you and allow you to test it (30 day guarantee)?
Speaking of, batteries are guaranteed from the manufacturer. I believe that guarantee starts on the build date of the battery and not your buy date. Just something to check into now that you are 3/4 of a year into it.
My personal preference is not to buy a battery more than 3 months old, but I buy flooded and they self-discharge faster than AGM.
DW, 3 Kids and our Goldens
2012 Shamrock 233S
2008 Toyota Sequoia 5.7L 4WD