Mearls -- I have no idea where he is right now. I think WotC probably didn't think firing him made sense (for reasons I do not know*) at the time of the controversy, and they certainly aren't going to either give him the axe or parade him out in public at this point. Like many things that happen within corporations where internal dealings are internal, we likely will never know all the specifics.
*he could have been valuable, they didn't think they had (lawsuit-proof) cause, they believed he'd done nothing wrong/made only reasonable mistakes, or any number of other potential reasons
The youthening of (D&D-side) WotC -- in 2013-4 the D&D team was a skeleton crew, trying to get this unifying edition out the door as an evergreen product, supposedly mostly with the mindset of it supporting the larger goal of themed merchandizing and D&D-related media. There would have been minimal support staff and creatives who cut their teeth in the late 2e and 3e era. Almost by definition there was a greyward tilt to the demographic, since vaguely 50% of the staff had 'and has many, many, (many) years of experience within the existing TTRPG industry' as a job-qualification. That's where the selective event would have taken place -- the shrinking of the D&D division creating a bottlenecking effect. In the intervening 8-9 years, D&D has wildly expanded, and the requisite staff has done so at the same pace. New creatives are being brought on, and while work experience in the field is part of that job description, they won't all be 'just the cream of the crop of the last few versions.' Likewise, the support staff, marketing and media, youtube presenters and so forth can be any age (with some fields, like said youtube people, having low-ward age demographics in general). This is going to drag the average age of the D&D-WotC employee down closer to the 'average white collar employed individual seeking new employment' age (which itself is lowering, as boomers stick in one spot until retirement and Millenials-GenZers are the new and mobile job-seekers).
Long story short -- it was when D&D went from 4e to proto-5e that the selection event which lead to a specific age range happened, and further activity is a regression towards the mean.