It is hard to tell if I have slowed down that much. As they say "with age comes knowledge ". I find that my bikes are maintained better than they were when I was younger. It used to be jump on the bike and off I would go. Brakes rubbing , tyres a bit flat and wheels out of true, which would all help to slow me down.
Now my bikes are maintained better and are easier to ride but other things have altered. Yes I have aged and I'm over 60 and I am on a few tablets but the road conditions have deteriorated so much that a similar distance has now increased over the years due to avoiding pot holes and rough areas. Rolling resistance has also increased due to the way that road surfaces are just treated with wet tar and loose chippings. It is amazing how fast you can go on a lovely smooth tarmac surface.
I may not have the stamina that I had when I was younger, but I was never any good at long distances. I find that my mind is too busy finding interesting things to look at . I ' m too busy looking over hedgerows looking at strange features in the landscape, stopping to look at the wildlife, or turning round in the road and going back to look at something that had fallen off a vehicle and ended up in the gutter.
So as far as I can tell I can still average about 10 to 12 mph. Which would include a couple of stops to get my breath back, which I would still have done in the 90's.