Older people tend to be wiser too.
What's the point of doing 55 in a 35 in a city with pedestrians and the like around. One misstep and they have issues to deal with that are easy to avoid by driving 35. With lights and traffic, 55 is unlikely to really save them that much time. Not worth the risk/reward.
What's the point of doing more than 65 on a congested highway? 65 will get you there in about the same time as zoom stop zig zag zoom stop zoom stop and be more pleasant, save fuel, and not call attention from the police.
If they want to drive fast they can take their track car to the track or their sports car to the curvy mountain road. When they were younger, they've probably done 55 in the city 70+ on their highway commute. They've realized it's not really worth it; there are other outlets of "spirited driving".
I've come to realize the same as I got older. I used to own a small sports car and drive my commute as spiritedly as I could. Now, I realize my commute in a large German sedan at a leisurely pace is maybe 5 minutes longer but far more pleasant and relaxing. I travel to the alps for a fun drive, or a coastal cliff-side road, or a track.