In reply to alexjz:
Apparently the average grade in 1991 was VS and in 2013, HS.
Interestingly if you look at sport, the average grade in 1991 was 6c and in 2013, 6a+.
There are a multitude of reasons for this. UKC Logbooks did not exist in 1991, so all recorded ascents are added retrospectivly. This would suggest they are by fairly dedicated climbers who are generally still active now. Amongst this user group memorable climbs that seem worth adding after the event are likely to be those in the higher end of each individuals usual grade range - ask me in 20 years about my climbs from 2010-2014 and I know the E grade routes will still be in the memory bank, some non descript V Diffs have already faded from my mind and I only know I climbed them due to conscientious Logbook compilation.
Obviously the dataset from 1991 is tiny compared to 2013.
In terms of Sport there were limited options in the easier grades 20+ years ago, which again will have effected the averages that get churned out by the omnipotent UKC database.
With the wider availability of good training facilities and a greater number of participants than 20 years ago I am convinced there are more people operating in the Trad E grades/Sport 7s than ever before.
My best guess is that the average Trad grade climbed has remained pretty constant once the bias factors have been ironed out. Sport average may have dropped due to availability of easier routes and more casual climbers focussing on this discipline.
Post edited at 16:10