Well, in theory, it could have undergone a malolactic reaction where some of the more acidic acids are transformed into less acidic ones.
And I'm really not trying to suggest anything derogatory about your palate, but are you sure it's acidic you tasted, and not bitter? I don't know how to explain the difference (and my husband always describes acidic things as bitter), but bitterness CAN age out, it does with JAO and I've got one batch that's about 6 years old now that is just starting to be palatable, it was spit-it-out-now bitter when I first made it, maybe another 5 years it'll be good (Patient? Lazy? I'll never tell!)
"The main ingredient needed is 'time' followed closely by 'patience'." - The Bishop 2013
"Good grief! If someone wanted to murder you, all they would have to do is ship you a 55 gallon barrel of honey and watch you document working yourself to death!" - Vance G
"When you consider that laziness and procrastination are the fundamentals of great mead, it is a miracle that the mazer cup happens." Medsen Fey, 2014