The risks and benefits are less cut and dry with females, largely due to the risk of pyometra, though with treatment the mortality rate of pyometra is pretty low. I wouldn't neuter a male, as they get almost no health benefits and it increases the risk of various health problems. With females it's more complicated, as spaying does reduce the risk of mammary tumors and almost eliminates the risk of pyometra. Spaying, even at 18 months or older, does still have downsides, therefore I like to make that decision for myself.
A study of Golden Retrievers found that even females spayed at or after 12 months of age were still more likely to suffer from mast cell tumors and hemangiosarcoma. Another study done on female Rottweilers found that not spaying until at least 6 years of age significantly increased lifespan. There was a study done recently comparing the lifespan of dogs in Europe with their American counterparts, it found that the European dogs had significantly longer lifespans. The main difference they found was that spaying/neutering is pretty uncommon in Europe, while being pretty much the norm in the US. Several studies have found that spaying/neutering at any age greatly increases the risk of hypothyroidism and various orthopedic problems, and even increased adverse reactions to vaccinations. Large breed dogs are at higher risk of spay incontinence, some studies estimate the risk could be as high as 20%-30%, even higher in some breeds that are more prone to it. Here's some links to articles referencing various studies.
http://www.chercarkennels.net/rethinkingspayneuter.html
http://www.naiaonline.org/pdfs/LongTermHealthEffectsOfSpayNeuterInDogs.pdf
http://healthypets.mercola.com/site...ce-your-pets-lifespan-by-over-30-percent.aspx