You can as easily argue the opposite: older people can be more conscientious because they have less to gain and less to lose. Who cares more about a payoff: someone who already has their gains and fewer years left, or the hungrier younger person?
I suspect you have based your “fact” on sampling bias - we don’t get good visibility metrics on young corrupted politicians (not so many young politicians, not so much time for them to have been caught making egregious errors). Disclaimer: middle-aged engineer type.
How many people in their tweens to 40th have ever filled the roles they were talking about? That's the only timeframe this argument would make sense to me.
I really can't see their comment as bigoted as you claim. There really is a strong selection bias with people filling these roles at that age and them behaving like that. That's doesn't mean that people in that age braket are always corrupt, but people that have been in these roles for extended periods of time generally are corrupt.
Representatives, whether young or old, never face the consequences of their actions because they exist in a separate system from the rest of society. They have special health care, private schools, and so on.