The only way to know for sure, unless they're obviously not healthy, is to test them. It's probable that they're at least somewhat worn, certainly, but if they have sufficient capacity for your needs and are otherwise working well, then replacement seems to me to be premature. On the other hand, if they aren't holding a charge, then you know that answer already....
Most 6V golf cart batteries are at least decent quality. If your electric power needs are relatively modest, I would probably look for the least expensive 6V batteries I could find locally, which may be at Sam's Club or Costco or some similar store. (There are not too many companies that actually manufacture lead-acid batteries, so many brands differ from each other mainly in the sticker that is attached to the side.)