The electric water heater in my cottage is 24 years old. It is only has
water in it 5 months a year, and is only turned on about 30 days in those 5
months. I replaced the heating element about 5 years ago, and drain it
yearly (obviously).
It is on a well, and probably has lots of sediment in it; I have cleaned
plenty of tiny stones out of the shower head and sink aerator..

I know water heaters only last 10 or 15 years, but is that years of having
water in them, turned on, or what?

On the one hand, I hate to spend money on something that is used so
infrequently, but on the other hand, if it starts to leak, it could be a
week or two before I find out!

Thanks.