They didn’t conclude that there were no mechanical problems with the aircraft. They just couldn’t find any evidence of such.
From the report, it sounds like the pilot was having difficult even before taking off, so it doesn’t sound like a payne stewart decompression incident. It sounds more like a possible stroke, or just general brain fog.
I wouldn’t worry too much about the FAA instituting a recreational pilot “retirement” age, though. The private insurance industry is more likely to do that anyway (which they effectively have already done by refusing to insure pilots over a certain age).