According to the rules, I believe you must have a senior staying in the room to qualify for the rate. In practice, I have frequently used senior rates when travelling with my mother (who is more than 62yrs), checked in with her waiting in the car (thus not visible to the FDC), and have never been challenged or asked for ID. This means that either they're pretty casual with applying the age requirement, or I look a lot older than my age...

I think the fact that it was an employee of the hotel that gave you the rate makes your case stronger.