Another car rental thread - car class and age restrictions

Hey All,

Time for another car rental thread as I have a question which I'm hoping someone can answer.
I was looking at a car for Dublin Airport from Sixt around the christmas.
As I'm only going to be 27 then that means Luxury class is out of the question according to the terms yet I still have the option to book it.
I usually go premium class and have no issues with this company which would be the step down from luxury class.
When I was searching the dates, for some odd reason luxury seems to be cheaper than premium despite being the better class.
My question is, if I was to book luxury class anyways (assuming the booking system lets me, looks like it will) and I show up to the counter, another assumption is that they will spot the age problem, will they just downgrade the car class or cancel the rental.
If I was downgraded, I'd have no problem really assuming I got the next step down which would be premium although I'd just hope they didn't see anything and gave me luxury :cool:

Just looking to see if anyone has any personal experience here.

Thanks

Tommy