I changed mine last year. I think they were five years old judging by the dates printed on the batteries.
They just weren't holding the charge very well. If I ran the fridge and radio it would die in a couple hours. Until I replaced the batteries, I always kept one battery for starting when it was time to go home and never had a problem.
I purchased my batteries at Costco. They were about $10-$15 each cheaper than batteries at West Marine. So far they are good batteries.