The National Highway Traffic Safety Administration created the training manuals for the field sobriety exercises. In their original 1977 and 1983 studies, they determined that standardized field sobriety tests were unreliable for people over age 60. This is reflected in the National Highway Traffic Safety Administration manual from 1991. In 2006 it was changed to age 65, however, this was not based upon any additional data. (Luckily, I still have the old manual to use during trial).
A recent British study indicates that the breakpoint for reliability on field sobriety tests is the age of 40, (Philip R. Dixon, Tony Clark, Brian Tiplady, Evaluation of a Road Side Impairment Test Device Using Alcohol, Accident Analysis and Prevention, 41 (2009) 412-418). This is more consistent from a medical perspective (think about it, there aren't many professional athletes in their 40's). Therefore, we may be able to argue that field sobriety tests should not be relied upon in your case because of your age.