What should the original post have said? Well, perhaps a framing that it was a personal - emotive - response to incidents witnessed, a level of frustration at the lack of institutional structure / process to contain or address such incidents, and some acknowledgement that the behaviour - though visible - of some individuals in a group doesn't necessarily extend to all members of the group would have been a start. But then, sweeping statements with disclaimers are not sweeping statements, and when one is in the heat of sweeping statement mode, rational thought and sober disclaimers don't occur to one. So yes, it no doubt could have been done a good deal better.
What should the follow-up have said? That I'm not the ageist I'm now being assumed to be? Well, I'm not entirely sure that would be honest. I am pretty ageist - though the target of that ageism shifts constantly. Sometimes it's older people, sometimes it's younger people, and while it's mostly probably people of my own age I'm dissing, I suspect the age group that chases up my blood pressure the most is babies. When colleagues pop in to display proudly what they've produced on parental leave, I turn tail and flee. I'm not the most tolerant, or the most patient, person I've met.
But then, age is not the usual focus for my sweeping generalisations. More often, it would be gender - though again, the sexism shifts with males and females in turn on the receiving end. Class is another frequent focus - though I tend, in this regard, to ascribe more agency to the wealthier classes (if they wanted, they could more easily shed their wealth than the disadvantaged classes could shed their disadvantage) and so more frequently vent in their direction. I have prejudices about language, about location, about religion, about sport, and about eye colour. And, while they might not be the standard ones, I nonetheless have prejudices about race.
All of which make navigating the world both challenging and interesting - and no doubt highly frustrating for those who have to share the planet with me.