Well technically, one of them didn't still work
The ages of the batteries are just a guess. We've only had the boat
for about a month, and it sat for the past 1.5 years pretty much unused. The guy we bought the boat from said he thought the previous owner replaced the batteries in '06 and he hadn't replaced any himself (bought the boat in '08). None of the batteries have a 6 punched out for the sale
date, 2 have 9's punched out and one has a 3 punched out. Since he said he didn't put new batteries in himself, I can only assume the 9's are from 99 and the 3 is from '03. I think that's almost absurdly old for batteries, but I have nothing else to go on.
Anyway, thanks for all the replies everyone.