Yes, it was a medical examiner, albeit one who was a local gynecologist. The tipping point for me with the age likely being accurate is someone in the initial thread--I think it was Tony--went to lengths to speak with Faces and got their opinion as to whether the age estimate was accurate.
I know on a few occasions with cases in the 60s, 70s, etc the ages have been widely off, but there are other things here that also make this seem more like a teenager to me than a young adult.
Something that has not been mentioned yet, and I think we may be viewing through a more modern lens: in 1975 at 19, 20, 22, 23? You were an ADULT, unless you had some sort of delay. Not that your parents wouldn't have loved you, but I do not personally think your suicide note would have been as directed towards a "missing son". We know law enforcement wasn't even particularly sympathetic in many cases towards families who tried to file missing reports for those 18+, right?
I don't think the note from an adult would have been as "Dear Mom and Dad" written by someone the age of Bayard Cousins.... He was a college grad already, right? (Or do I have this confused?). I know he left the "Don't worry about me" note, but that is also consistent with a young adult who is just taking off. Maybe his parents were controlling/overprotective or it felt that way to him, and he left for somewhere to engage as an activist?
Where Charlie Wallace was involved in drugs, he certainly could have crossed the wrong people as well. Owed someone money, or overdosed somewhere and others got rid of his body.