Final answer:
The rational basis test is the standard used for age-based equal protection cases, being the least stringent form of judicial review and placing the burden of proof on the challengers to show no good reason for the differential treatment.
Step-by-step explanation:
The correct standard used to determine constitutionality in age-based equal protection cases is the rational basis test. This test is the least stringent form of judicial review used by the courts in considering Equal Protection Clause issues. Under this standard, the burden of proof falls on those challenging the law or action to demonstrate that there is no good reason for treating them differently from other citizens. Age-based discrimination generally does not involve a suspect classification that would warrant a higher level of scrutiny, unlike race or gender, which are subject to strict scrutiny and intermediate scrutiny, respectively. For instance, the courts have ruled that government actions that discriminate on the basis of gender must meet the intermediate scrutiny standard by showing that the action is substantially related to an important governmental objective. On the other hand, race-based discrimination is subjected to strict scrutiny, requiring a compelling governmental interest and the action being narrowly tailored to achieve that interest.