I believe it says "5% chance" not to age...meaning it is basically random. Five times out of a hundred it shouldn't age...but then again, if you flip a coin 100 times the averages say 50 of those 100 it will be heads but go ahead and DO IT, and you'll very rarely get a 50/50 split.
I had one horse who didn't age for like four turns, and another who only didn't age once in the entire time I trained him. It seems pretty random.
I doubt it is a bug, just bad luck.