Ridiculous, they shouldn’t be on the road if their reactions are slow- it isn’t safe!
Not everyone who reacts slower is unsafe unless we say that the average adult reaction time is exactly on the cusp of what is safe and what isn't.
If I can react in 0.3 seconds and someone older than me can react in 0.6. Where between 0.3 and 0.6 is the safe line?
I would suggest both are safe, though one is slower than the other.
We already accept that different groups of people have variation in reaction times, e.g. males have a faster reaction time than females. Yet both are allowed to drive. People with active lifestyles have faster reactions than sedentary people. Yet both drive.
There is obviously a cut off at which point it becomes unsafe but before that there is variation between reaction times that are within a standard we (society) considers to be worth the risk. Whilst I hate these kinds of signs, I do think there is a place for faster drivers having some patience for slower ones in the understanding that being able to drive brings that person personal benefit and probably benefits wider society to a degree.
If you need concessions from other drivers then you shouldn't be driving.
This is always easier to say when it is someone else. Let's make an assumption that you are female with average reaction times for a female. These are slower than the average reaction times for males or athletes. Therefore, aren't you getting concessions from them when driving? Or maybe you didn't get quite as much sleep last night as normal. This would slow down your reaction. Or maybe you have a cold but still need to work. That would slow you down as well. None of us operate at our very highest standard every minute if every day. Most of us would have gotten away with silly mistakes because the other driver was quicker on the brakes or because they spotted the potential for trouble before we did. So, at some point, all of us get concessions.