An individual 40 years or older are over the age for reliable SFST’s. The NHTSA, based upon studies done in 1977 and 1983, determined that SFST’s were unreliable in those over age 60. This is reflected in the NHTSA manual from 1991. In 2006 it was changed to age 65, however, this was not based upon any additional empirical data.
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. Therefore, psychophysical cannot be relied upon in people greater than 40 years of age.