Probably a little soon, but not out of line. Li-ion dies from two things: Age and charge cycles. They start losing capacity pretty fast after about three years and it's a function of the chemistry. That's probably what's in play here. See if you can find a date code on the battery - they usually have the month and date encoded in a number. That's going to matter more than how long you've owned it. For example, if you bought the bike but the battery had been manufactured a year before you got the bike, then you'd be right on the schedule for it's aging issues.

The other thing is if you keep the bike out in the garage during the winter and it's freezing or getting really hot (or worse, both), you'll age the battery faster. There are methods for testing component life by accelerating the aging and they all involve temp cycles. So if you, say, live in a place where it gets really cold in the winter and your garage goes down below freezing, you are going to prematurely age the battery through freezing.

Either way, it's time for a new battery.

J.