Senior discounts are provided by insurance companies in an attempt to win business over other insurance companies. Like all other forms of auto insurance, a senior discount auto insurance policy covers the car, not the driver. Therefore, if the senior citizen lends their car to an individual for short-term use, then the younger driver is fully covered under the insurance policy, as long as they are legally able to drive.
The one factor to keep in mind is that the auto insurance provider is insuring the car based on the assumption that the senior citizen is the primary driver. While the senior citizen is able to lend the car to others, it should not be done so for long periods of time. If the auto insurance provider determines that the younger driver is actually the primary driver then you could be in violation of the insurance contract and the whole policy could be void.