I'm 76 and have been riding for about 40 yrs. Over the years I've lost significant power and gained that 10 lbs I can't lose. All pretty normal so I don't expect my MPH today to be what they were then. Having said that, it's important to remember that on any ride (s) there are many variables such as elevation, wind, air density, temperature, etc. that are going to affect your ride. Unless every variable is the same from one ride to another it will be impossible to make 100% accurate comparison. OTOH you can see trends for a general idea.