What's the lifespan of an X5 battery?

[DaveNW]

I have an '03 X5 4.4i I just purchased a few months ago. It seems now that the weather is colder here near Seattle, it's seems harder to start first thing in the morning. I turn the key and there seems to be a delay before it cranks - and the cranking seems a bit sluggish.

I don't know squat about batteries, and I'm wondering if my (presumed) factory original battery is wearing out? I don't know if this is normal X5 behavior on a frosty 30F morning, or signs of a potential future problem. What I most want to avoid is a day when I turn the key and get nothing back from the X.

Anyone have anything to offer about this?

Thanks,
Dave