There are many good articles in a stickie thread that Snowshoe compiled. Neuter/Spay - Pros, Cons, Risks, Benefits - Research Article Links
What did your breeder suggest? The 2 breeders from whom I've gotten my labs definitely had their own recommendations. FYI, Chase, now 7, was neutered at age 3. Lark, age 18 months, will be spayed in about 2 weeks, she's had one heat cycle. Chase has always been a sweetheart and that didn't change with neutering. The vet said he might not metabolize calories the way he had previously, but limiting his intake and trying to get him to exercise some have kept his weight between 85-90 pounds. I think PopTop has had some "older" dogs neutered, she only adopts seniors or older adults.