7 years amazes me too but that's what I get. I have had my 208 for 14 years and I am on my second set. Towards the end of last season, I did notice that one battery is getting a bit tired. If I ever get that boat in the water this season, it will probably need new batteries. For the record. I leave my batteries in all winter. I make sure they are fulling charged before storage. I disconnect the grounds and do not trickle charge during the winter. The motor has started every new season so far
In the case of parallel wired batteries, I can see how they may age faster depending on how 'isolated' they are. Often one battery draws a tad from the other while the other 'charges' its twin. In addition depending again on isolation and the type of charger, the shore charger can in theory overcook a battery if its twin requires charge but it doesn't.
Finally, batteries in warmer climates ( think Florida) age faster than those in the cooler northeast