May or many not be representative to control just for age. There are a number of environmental variables like elevation change, wind speed /direction, equipment, etc. that might make the same distance over time still relatively meaningless as a comparison.
I ride with 30 year olds who struggle with your 17.5 mph average on flat ground. I occasionally ride with a group of 60-70+ year olds, some of whom are comfortable with that or faster in rolling hills terrain. Most of them have been riding for many years.
Non-competitive environments such as recreational or fitness group rides, might give some indication, but most group rides have a specified performance target. Find one where you fit in comfortably and your limits (and those of others) probably won't be challenged. Or find one where you "just" to fit in and still the limits of others might not be challenged even though you are all riding as a pack. Adding to the complexity is that group rides are almost never segmented based on age, but rather by distance vs. time (average speed) objectives. You might find a weekday early/mid morning ride group that "self-selects" for older riders just because younger cyclists are working and not retired/semi-retired. Whether the group focus is on fitness or camaraderie will determine if it provides any relevant performance data for you.
Semi-competitive environments like Gran Fondos usually segment results by age bracket. Some participants go all out in Fondo events and others are out for a "spirited ride". Find some Gran Fondo results for a ride in your area and ride the same course to see how your time stacks up. The caveat is that many Fondos time a few key climbs as the competition factor and not the entire event.
Another option would be to sign up for MapMyRide.com or Garmin Connect (or other data collection site) and identify some local cyclists in your general age range and ride the same routes they have done and compare distance vs. time. That is still heavily dependent upon environmental factors and the amount of effort the individual intended to exert as a ride goal.