Since you have asked three times and nobody has answered I'll...

Since you have asked three times and nobody has answered I'll give it a crack for you WBA.

Short answer is ethics considerations.

When people are 60+ they are considered aged and vulnerable. Take no offense. Its mere categorization. More importantly they are considered high risk for severe reactions to Covid. This is why nursing homes etc were given high priority during the vaccination efforts and lock-downs.

Participating in drug trials comes with additional inherent risk. From an ethics perspective its unnecessary to experiment on high risk patients when they are already exposed to a virus which is potentially lethal to people in their cohort so adding an experimental drug with other potential side effects is an unnecessary ethical risk given that there are ample numbers of lower risk patients to conduct these safety and efficacy experiments on.

Does this help?