Isn't that just a case of it being more popular making the mean average slower? My club has a hell of a lot of middle aged slower runners who've joined over the last few years but the best are still as fast as anyone who came before them I wouldve thought
I`d say that the majority of trail races I do, are populated by the 40/50 age group, both men and women, for the simple reasons that trail running in particular has taken off amongst the white / middle aged / middle class, who have time on their hands due to their kids being grown up and the fact they can afford to have weekends away in the likes of Cumbria / North Wales / Yorkshire etc and compete in races, that previously were the preserve of serious competitors from running clubs.
These running clubs still have serious and elite runners, but they also have a lot of nuts and bolts runners, who just enjoy the day out and the social side of things.
I`ve got mates who cycle and cycling is the same apparently.
I don`t road run, but I should imagine that road races are the same too.
So I`d say your pretty much spot on about the mean average becoming slower, as more " older fun and social " runners are now bulking up races.