I get a new set every 3 years, just to have the peace of mind of having fully charged, healthy batteries. Believe me, it only takes one time that you can't start because you "stretched" out your battery life and there's not enough juice to crank the motor over. I speak from experience.
I also take mine out at the end of the season, wire them together and put on a trickle charger for the winter.
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.
My batteries came with my boat 5 years ago. I could tell by looking at them they were not new. I have no idea how old they are.
There still good.
I pull them out over the winter and put them in the garage on a battery maintainer / trickle charger.
I was thinking about replacing them this spring if I still have my boat.
Just to be safe.