There are some demographic studies that do seem to show age at commensing racing and age in general don't quite support the 'older is better' notion--or at least intensity of training is a far more influential factor. I am still tracking down 1st sources but the dissenting option is outlined in a sensible way here: Wenholz SD. Training: impression vs. research. Grayson-Jockey Club Res Today 2003;20:1-4 (available online). The experiments I have track down don't show a strong age of first race effect, and the overall age effect is that younger horse are at more risk in jumping races and older horses more at risk on the flat--e.g. Pinchbeck GL, Clegg PD, Proudman CJ et al. Horse injuries and racing practices in National Hunt racehorses in the UK: the results of a prospective cohort study. Vet J 2004;167:45-52.

emily: My read of Dr. Nunamaker's studies (which most of the jockey Club article you referenced cites) is that it's about length of training before racing rather than the optimal age to race a horse. This article clearly concludes the opposite based on the same research. It does mention one British demographic study and one Davis study (which are not discussed in detail) and though I'll look at everything with an open mind, I wonder if their conclusions aren't as hasty as they were for Nunamaker's research.

"Anything that slows down the racing speed will save some lives." - Emily

Hmmm, starting to sound like NASCAR. Too bad we can't put speed governors (sp?) on race horses. ;-) <just kidding> I believe that NASCAR and the other car racing sports create rules that hold the speeds down. It's been a while since I paid attention to that, but I wonder if there are any lessons to be learned there.

Dr K, thanks for the thoughtful post.