I think you missed the part where there were much much less software devs/engineers earlier.

Year after year it was just much more new people joining as things got easier and more accessible.

Now you see 40 or 50 year olds far and between where most guys I see are in their 30s. Ones that are 60 yo diluted in the sea of new entrants.

Ageism didn't came from the top it just happened with flood of young employees, there is just social dynamics where you might get 40yo not being a manager getting along with bunch of 25yolds but that's going to be an exception not the rule.