http://phys.org/news/2013-02-golden-...og-health.html

Important to keep in mind this is a single study and on a much larger breed than the cavalier -- and one far more prone to cancer and hip dysplasia (though hip dysplasia IS an issue in many smaller breeds too, including the cavalier). But some thought-provoking results that would suggest it may be better to neuter males after a year of age; while it may be better to spay females before a year old (setting aside the increased mammary tumour risk already known to occur in female dogs spayed after the 1st or second heat).