Normal? No. Unheard of or particularly unusual, nope. Batteries are strange critters. However you have committed the unpardonable sin by speaking about it. So you might as well go buy your new battery now....
Yeah, they last a long time, but not forever. According to the Yuasa representative that answers e-mails, all lead acid batteries begin to deteriorate as soon as they are filled.
I received another YTZ14S less than two weeks ago, put the Battery Tender Jr. on it, and in less that 15 seconds the light turned green. New AGM batteries have a self discharge rate of one percent per month while new flooded cell batteries have a self discharge rate of three to four percent per week.
The AGM battery in my Harley FXD is 66 months old. Up until this winter, it could sit for two months and start right up. Now if it sits for two months, it will not even turn the motor over. After charging, it starts good, but it is getting weaker.
The 67 month old AGM battery in my FLHR is getting weaker also, but it is a larger battery starting basically the same motor. I had never even seen the battery in the FLHR until this winter and never had a charger on it.