At 75 I feel qualified to say that there are two problems as far age goes. One, body parts don't do the same level of work and Two the meds you take will probably have a much bigger impact. For me: at 50 sub 5 100 mi, at 75 sub 7 100mi and I am now thinking just finishing 100 miles at 85 will be good enough. A few months ago I accidently stopped taking one of my meds containing statins. Next day and until mistake discovered my daily average speed went up by over 1 mph.